Salsa a la Diabla

• 4 Roma tomatoes, cut in half
• 2 cloves garlic, not peeled
• 1 yellow onion, cut crosswise into slices ½ inch (12 mm) thick

Preheat the oven to 350°F (175°C). Place the tomatoes, cut side up, on a sheet pan along with the garlic and onion slices and roast for about 25 minutes