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. Serve warm.