Sweet potatoes are my go-to taco punch-up. I was going my normal route of making small (1/4" or so) cubes of sweet potatoes when I thought: Hey, it's fall, I'm going to add an apple in there. Fuck yes.
Mix this all into a pan, with about 1/4 cup of water, and boil it all up until the water goes away, toss in the butter, and continue to pan-fry until things get a little browned.