Preheat the oven the 400 degrees F. Line a large, rimmed baking sheet with unbleached parchment paper.
Put the potatoes, onions and olive oil in a large bowl and toss to coat. Put the chili powder, garlic powder, paprika and cayenne pepper in a small bowl and stir to combine.
Add the spice mixture to the potatoes and toss gently to coat.
Spread the potatoes and onions in a single layer on the prepared baking sheet. Bake for 45 to 55 minutes, or until soft on the inside and golden on the outside, turning with a spatula halfway through baking.
Cool slightly and add more salt, as desired. Serve warm.