Baking – How does foil protect a sandwich in the [toaster] oven

If you're worried about the aluminum foil, you can always just use a covered casserole dish ... it might not hold all of the steam in, though. (there are methods, like mixing a pough of flour and water, then using that to seal the lid on the pot).

Also, if you're worried, I'd just consider not using acids, like lemon; aluminum pots and pans react to acids, and I'd only assume aluminum foil would to.

You could also switch to parchment paper, which is the original for cooking 'en papillote' (in paper).

But 'better' is subjective -- easier to clean up (aluminum foil wins, I think), cheap (maybe the casserole dish, as it's reusable), more impressive (probably the parchment paper).

As for spicing ... search for 'chicken en papillote' on any internet serch engine and you'll find lots of recipes.

Possible reasons:

• Horrible temperature control - this is simply a problem of cheap construction. One could theoretically build an excellent toaster oven with precise temperature control, but why would you?
• Low thermal mass - If you get an oven to 400F and open it for long enough to put in food, the result is an oven at very slightly less than 400F, and which will quickly return to 400F. Do that with a toaster oven, and who knows what temperature you'll get, or how quickly it will get to your desired temperature (if ever, see #1). This is not a slam at toaster ovens, this is just due to them being approximately 2% of the volume of a standard oven.
• Closeness to elements. It's easy for toaster ovens interiors to vary by nearly 50F from middle to edge, simply based on distance to elements

Now don't get me wrong, I once made some kickin' Oysters Rockefeller in a toaster oven, when I thought that they would be the difference between serving an appetizer and being allowed to serve breakfast the next morning (wink, wink). If you know what you're doing, and watch carefully and continually, a toaster oven can be an excellent source of high(ish) direct heat. But they are extremely difficult to control, and extremely unforgiving of errors. I can't think of a circumstance where a toaster oven would be my first choice of cooking tool (and that includes microwave, campfire, plumber's blowtorch).