Preheat oven to 400° Line a large baking sheet with parchment paper.
Cut tomatoes in half lengthwise
4 cups fresh tomatoes
Toss tomatoes & quartered onions In olive oil and spread in a single layer over the baking sheet ( you may need two).
2 onions, ¼ cup olive oil, Salt & Freshly ground pepper
Cut the top off of a head of garlic, drizzle with a little of the oil, and put it onto the baking sheet.
1 head of garlic
Roast everything for about 30 minutes, until the vegetables are tender and starting to turn golden brown.
Put the vegetable stock, canned tomatoes, basil, and roasted tomatoes and onions, along with their juices, in a large pot or large Dutch oven.
4 cups vegetable stock, 1 28 oz 796 ml can Italian plum tomatoes, with juice, no salt added, 3 cups chopped fresh basil
Squeeze out the roasted garlic into the soup pot.
Bring to a boil over medium heat, then reduce heat and simmer for 10-15 minutes.
Use an immersion blender to puree the hot soup right in the pot, or, transfer in batches to a blender or food processor, remembering to open the steam vent.
Add salt and pepper to taste. Garnish with more fresh basil.
Notes
Roasting Tomatoes: This is an extra step but takes little effort and it's absolutely worthwhile for the rich flavors it brings out.
Blending: My favorite way to puree soups is right in the pot using an immersion blender, which is super handy to have around the kitchen for all kinds of things. If you don't have one, you can use a blender or food processor. Transfer the soup to the blender (in batches, depending on the size of your blender). Allow the soup to cool somewhat first, and be sure to keep the lid vent open to allow steam to escape and prevent a soup explosion.
Consistancy: After blending you can thin the soup with more broth if it's too thick for you. And if you want a smoother soup, strain the soup though a colander or strainer.
Tomato Paste: If you don't have fresh garden tomatoes, you can still make this great recipe with grocery store tomatoes. However they are often not as ripe as they could be. A tip for maximizing their flavor is to stir some tomato paste into the soup along with the vegetable stock and canned tomatoes.
Storing the Soup: Roasted tomato soup is even better the next day. Cool the soup completely, and store it in an airtight container in the fridge. It will keep 4-5 days. Reheat on the stovetop or in the microwave.
Freezing: Let the soup cool completely. Portion it into freezer-friendly airtight containers and store for up to 3 months. Defrost in the fridge.