Preheat oven to 400°F (200°C). Place tomatoes, onion, and garlic head (top sliced off) on a baking sheet. Drizzle with olive oil, sprinkle with salt, Italian seasoning, and red pepper flakes. Toss everything directly on the baking sheet. Roast for 35 minutes, or until soft and caramelized.
2 pounds ripe tomatoes, halved, 1 large yellow onion, roughly chopped, 1 head garlic, top sliced off, 3 tbsp olive oil, 1 tsp Italian seasoning, 1 tsp salt, or to taste, ¼ tsp red pepper flakes, or to taste