I'd say pasta, rice and potatoes are standard starting places for when I find myself in the kitchen with no idea what I'm going to make! Leftover veggies and meat from previous meals can be thrown in or on top of or next to any one of those, with some butter and/or olive oil and seasonings. Frozen veggies and/or canned tuna are handy if there aren't any leftovers and I'm not in the mood to cook fresh meat or veggies.

And of course sauteed onions are always a good thing! But I can't tell you how many times I've tossed a dish together and then suddenly realized, "Oh, I should have sauteed some onions!" So it sounds like your starting point is something I should always keep in mind, yes? :-)