Heat oven to 425 degrees F.
Toss together potatoes, garlic, salt, pepper and olive oil in a large bowl to coat the potatoes evenly with the oil and seasonings.
Spread the chopped onion and diced tomatoes over the bottom of a large roasting pan. Distribute the potatoes evenly over the top of onions and tomatoes in the pan.
Roast in 425 degree F oven for about 40 minutes or until the potatoes are almost fork-tender, stirring the mixture 2 or 3 times with a wooden spoon during the cooking.
Sprinkle the capers evenly over top of the potatoes. Roast the whole mixture in the 425 degree F oven for an additional 15 minutes or until the potatoes are fork-tender. Makes 8 servings.