Preheat oven to 375-450°F (190-232°C).

Prepare tomatoes: peel and core if desired, then halve, quarter, or cube into 1-inch pieces to measure 4 cups. If using cherry tomatoes, remove stems.

Place tomatoes in a large roasting pan, jelly-roll pan, or any baking sheet with sides.

Add onion chunks, chopped basil, and peeled garlic cloves to the roasting pan.

Ensure vegetables are in a single layer, but crowding is acceptable.

Drizzle with olive oil, then sprinkle with salt and pepper to taste.

Roast until tomatoes turn a deep golden brown, 20-90 minutes, depending on oven temperature.

Tomatoes will collapse and look wrinkled when done.

Alternatively, roast over indirect heat on a grill with the lid on for a smoky flavor.

Remove pan from oven or grill and let cool.

Scrape the pan with a metal or wooden spatula to dissolve cooked-on bits.

Optional: Peel tomato skins after roasting (skins come off easily).

Optional: Blend the roasted tomatoes and pan juices in a blender or food processor until pureed.

Stir the puree back into the vegetables before refrigerating or freezing.

For peeling tomatoes before roasting: Cut a shallow X on the bottom of each tomato.

Blanch in boiling water for 30 seconds, then remove and cool under cold water.

Peel the skins off, then core and cut as directed.

To freeze: Ladle the sauce into freezer containers or bags, leaving 1-inch head space.

Let cool completely, then close and freeze.