Heat oven to 400 degrees.
Place the prepared potatoes (or yams) and onions in a reusable plastic bag.
Drizzle in the olive oil.
Shake bag to coat vegetables well.
Next, spread the vegetables into a glass baking dish in one layer; sprinkle evenly with the chili powder.
Cover with foil and roast for 35 minutes.
Remove the foil, carefully add the chopped garlic evenly, then cook for 10-15 minutes, stirring until the vegetables are tender and lightly browned.