How to fry leftover mashed potato

It sounds to me like the issue may be that you're crowding the pan.

Basically, to get everything nice and brown and crispy, you need enough space for all of the steam to escape. That picture you showed has potatoes stacked on top of each other -- that means as the bottom items cook, they're going to end up steaming the items above them.

At a diner, they have a large griddle to work with -- they can really spread things out. You're not typically that lucky in a regular kitchen, as you don't have as much space, and you have a lip on the pans that'll hold the steam in.

So, either work in smaller batches, or consider recipes that use an oven -- using sheet pans instead of a pan on the stove solves much of the problem.

One other trick is that most diners don't start from raw potatoes -- maybe with hash browns, but not for home fries, you're not going to get the nice soft interior in a reasonable amount of time unless you start with a potato that's already been baked or boiled. (If you're doing things in the oven, you might be able to, but not in a pan)

Just for reference ... I have a 14" cast iron skillet that I use for home fries ... and it's about the right size for cooking a single large potato, which might be two servings, maybe three for kids. (I tend to cook carb-heavy meals).

update : I probably should've stated this directly -- you want the chunks of potato to form a single layer in the pan, with space in between them.

Your dealing with a few things here. First is starches in general. The thing to know about starches is how they gelatinize and at what temperatures.

This powerpoint is a nice primer on that topic. www.cfs.purdue.edu/class/f&n630/gelatinization.ppt

Basically your dealing with amylose and amylopectin, together they are what we know as a starch. When they come in contact with water the starch cells begin to swell and when their gelatinization temperature is met they burst and release their contents into the medium they are in. In the case of a dough ball, your dealing with tons and tons of little cells being held together loosely at first by the physical pressure of kneading them into balls and then when the heat causes the starches to gelatinize they adhere to one another kind of like being caught in a net.

Now the fluffiness portion of the question can really depend on how the dough is being cooked. For something like a dumpling being cooked in the boiling water, one would typically want to work the dough as little as possible to avoid making it too dense and if using flour to lessen the gluten formation that can make it very chewy like bread. Some recipes call for leaveners that can create gas bubbles when heated to a certain temperature and then through gelatinization the bubbles are trapped inside the dough and create an airy texture.