[RPG] What types of food are typically grown in the Underdark

There is a specific Feywild for the Underdark, and it's called the Feydark

It is found in the Underdark Supplement for 4e.

Here is an excerpt about the Feydark.

It appears in Matt Colville's homebrew campaign, but not D&D canon

In his Campaign Diary 010, Matt Colville describes that in his setting the Underdark is not just physically underground, but somehow a dimensionally separate world:

He provides the Chain with what is basically an interdimensional map that they could use to get from this world to the World Below, which is where the adventure happens. In my setting, the "Underdark" is called the World Below, and it's another world; it's another realm.

Note that in a previous version of this answer I said that Matt had previously described his Underdark as if it was merely physically below the Earth. However, it's plausible that Matt has described his Underdark as another world before this question was asked, and it merely hadn't appeared in any of the videos or content I was aware of.

However, not in canon

The list of canonical Dungeons & Dragons books is finite, and of those, the only works likely to define the Underdark in any canonical sense are campaign setting books, setting-neutral adventures taking place in the Underdark, guidebooks to the Underdark, and magazine articles. Novels in a given setting are unlikely to contradict their sources.

Many settings either do not have a planar cosmology at all, or do not even have an Underdark, which was largely a Greyhawk-specific feature adopted by the Forgotten Realms in AD&D 2e and later settings from 3e onward.

This lets us narrow down the possible sources to something we can manage.

Setting-specific

We can generally assume that setting lore remains constant between editions.

• Greyhawk: The most recent source is the Living Greyhawk Gazetteer (2000), p. 18, which mentions "the subterranean Underdark", containing the Vault of the Drow physically located beneath the Hellfurnaces mountain range.
• Forgotten Realms: The 3e book Underdark describes it as a series of caverns "that riddle the earth beneath Faerûn". The 4e Forgotten Realms Campaign Guide describes it nearly identically as "a network of subterranean tunnels, caverns, seas, and rivers that spans the entire world". The 5e Sword Coast Adventurer's Guide, p.99, affirms that in the Forgotten Realms, the Underdark is still physically beneath the surface of the world. The Al-Qadim and Kara-Tur settings are parts of the Forgotten Realms.
• Eberron: According to the Eberron Campaign Setting, the Underdark is largely synonymous with Khyber, again, a place physically below the earth.
• Dark Sun: The 4e Dark Sun Campaign Setting mentions the Underdark only in passing, but describes on p. 116 that planar travel is rare, and the Underdark does not appear in its cosmology described on page 17. Its AD&D 2nd edition counterpart, the Dark Sun Campaign Setting, does not mention any Underdark.
• Ghostwalk: The Underdark is mentioned only once in passing.
• Ravenloft: Sequestered from the rest of the planes, Ravenloft effectively has no planar cosmology into which a Plane of Underdark could be placed.
• Hollow World: Spells for planar travel do not work, meaning that Hollow World effectively has no planar cosmology.
• Spelljammer, Planescape: These works expressly did not contradict Greyhawk or Realms planar cosmology.
• Nentir Vale: In the "World Axis" cosmology that was part of the nameless (but fan-nicknamed) core setting of 4th edition, the Underdark is portrayed as functioning very similarly to a transitive plane. Simply exploring its tunnels can bring you from or to any of the following planes: the mortal world (Material Plane), Feywild, Shadowfell, Elemental Chaos, Far Realm. It is not explicitly described as a transitive plane, but such terminology is not really used in the World Axis anyway.
• Other settings: Birthright, Dragonlance, Jakandar, Mystara and Rokugan either have no Underdark or have no established planar cosmology in which an Underdark could be placed.

Setting-neutral

• The AD&D Dungeoneer's Survival Guide asserts that the Underdark consists of subterranean caverns deep beneath the earth.
• The 3.5 Dungeon Master's Guide, p. 59, defines it as "a subterranean world that is one enormous natural cavern complex running under the surface of entire continents".
• The 4e Dungeon Master's Guide, p.106, defines the Underdark as a "sprawling networks of natural caverns stretching deep below the earth". This is canonical for the Points of Light setting and generic sources.
• The 5e Dungeon Master's Guide, p.101, describes that mining too deeply can break into the Underdark, which suggests that it's physically located on the material plane.
• Night Below insists that it has been written to integrate into any campaign setting. It suggests that the Underdark is located beneath the world.

Other sources

• The DragonDex lists all articles appearing in Dragon magazine which reference the Underdark. Of those, only one describes possible alternate Underdarks, and none of them are extraplanar in nature.
• As best I can search Wikipedia and Google, there is no reference to the Plane of Underdark.