Preheat the oven to 400 degrees F (200 degrees C). Spray a baking sheet sheet with cooking spray. Place garlic bulb and onion on the prepared sheet.
Roast until very tender, 40 to 45 minutes; remove from oven and let cool until cool enough to handle.
Chop half the onion; release 4 garlic cloves. Set aside remaining onion and garlic.
Heat olive oil in a Dutch oven over medium heat. Add chopped roasted onions and 4 roasted garlic cloves to the hot oil, and cook and stir until fragrant, up to 1 minute.
Add both cans of San Marzano tomatoes (including juice), vegetable stock, and agave syrup. Bring to a simmer and cook until thickened, about 15 minutes.
Carefully transfer tomatoes and some of the liquid, remaining onion, remaining garlic, fresh basil, and fresh oregano into a food processor or blender. Blend until there are no chunks.
Add soup mixture back into the Dutch oven along with heavy cream. Season with salt and pepper. Simmer on low to medium heat for 5 to 10 minutes to allow flavors to meld together.
