Leave cherry tomatoes whole. If using larger tomatoes, quarter them. Quarter the onion and separate the garlic into individual cloves.
10 Cups Tomatoes, 1 Medium Onion, 1 Medium Head Garlic
Arrange everything on the pan. Spread the tomatoes, onion, garlic, basil, and oregano evenly over a half-sheet pan.
½ Cup Fresh Basil, ¼ Cup Fresh Oregano
Drizzle the olive oil over the vegetables and sprinkle with salt and pepper.
2 Tablespoons Olive Oil, ½ Teaspoon Salt, ¼ Teaspoon Black Pepper
Roast for 25–30 minutes, checking after 25 minutes. The tomatoes should be significantly reduced and lightly browned around the edges without being burned.
Transfer the roasted vegetables and their juices to a blender or food processor. Blend until smooth, or pulse a few times for a chunkier sauce.
Serve or store. Serve the marinara warm or allow it to cool before refrigerating or freezing.