Preheat your oven to 425 degrees F and line a large baking sheet with foil to prevent the food from sticking to the baking sheet.
Add the tomatoes and onion to the pan and drizzle over 1 1/2 tablespoons of olive oil along with the balsamic vinegar (balsamic is optional, but adds more richness). Sprinkle to taste with salt and pepper and toss to combine.
Slice the top off the head of the garlic, making sure to leave the bulb intact. Drizzle the remaining half tablespoon of olive oil over the garlic and wrap it tightly in foil. Place the garlic cut side up on the baking sheet.
Roast for 35 minutes in the preheated oven, or until the tomatoes are caramelized and the onions are getting slightly golden brown.
Carefully remove the garlic from the foil and squeeze out the gloves, it will be hot. We like to let it sit for a few minutes to cool.
Add the garlic cloves to a food processor, along with the roasted tomatoes, roasted onion, remaining two tablespoons of balsamic vinegar (optional), and fresh basil. Puree to your desired consistency and season with salt and pepper to taste.
Enjoy now or refrigerate to enjoy within 3-4 days or freeze up to 6 months.
Notes
You should get about 2 cups total for 1/2 cup per serving.