When making baked potatoes, I never do anything other than wash them well, prick them with a fork a few times (otherwise the potato might explode in the oven from its steam), and throw them in the oven. I know many people oil them, but mine turn out just fine. I WILL wrap them in foil only if they’re going in the coals in the backyard grill.
Twice baked potatoes look fancier with only a silce off the top. However, I find it’s easier to simply cut them in half, the long way, and scoop out most of the innards. As others have noted, don’t use a mixer. I can add, don’t use a blender, either. You will NOT be happy with the results.
Twice baked potatoes MUST have the fillings mashed with cream, sour cream, or milk (preferably whole milk). After that, just put in whatever you like. I generally put in shredded cheddar or parmesan cheese, plenty of REAL butter, and chives. Sometimes I put in sauteed mushrooms or cooked crumbled bacon, too. I imagine you could put in chopped onions, raw or sauteed as you like them.
This is one of those “no recipe” dishes…I just put in enough milk or cream or sour cream to make the potatoes the right consistency. Pretty much the same consistency as mashed potatoes. I put in enough cheese to taste the cheese, I’ve never measured the amount.
When I bake potatoes, I generally bake six at a time, and we eat three for that meal, and have another three for nuking or making TBPs or soup or whatever. If I nuke a potato for myself, I’ll generally cut it in half, skin it, coarsely chop it, and then grate some cheese over it before I put it in the microwave. Then I top it with sour cream and a bit of salt, and maybe some bacon bits. It’s a very comforting lunch.