Preheat the oven to 400 degrees F (200 degrees C) and spray a baking sheet with cooking spray.

Place the garlic bulb and onion on the prepared sheet and roast until very tender, about 40 to 45 minutes. Remove from the oven and let cool.

Chop half of the roasted onion and release 4 roasted garlic cloves. Set aside the remaining onion and garlic.

In a Dutch oven, heat olive oil over medium heat. Add the chopped roasted onions and garlic cloves, cooking and stirring until fragrant, about 1 minute.

Add both cans of San Marzano tomatoes (including juice), vegetable stock, and agave syrup. Bring to a simmer and cook until thickened, approximately 15 minutes.

Transfer the tomatoes, remaining onion and garlic, fresh basil, and fresh oregano into a food processor or blender. Blend until smooth.

Return the soup mixture to the Dutch oven, add the heavy cream, and season with salt and pepper. Simmer on low to medium heat for 5 to 10 minutes to allow the flavors to meld together.