Preheat the oven to 425°F (220°C). Cut the onions and larger tomatoes into chunks. Place them on a large baking sheet with the cut side up. Keep the grape/cherry tomatoes whole and add them to the sheet pan. Drizzle olive oil and a little salt and pepper over the veggies.
Slice ⅓ off the end of a garlic bulb so that the cloves are exposed. No need to peel the bulb. Drizzle the head of garlic with 1/2 tbsp of olive oil. Loosely wrap it in foil, but make sure it’s completely sealed. Place it on the sheet with the cut side up.
Roast the vegetables for 40-45 minutes until they are slightly browned. Flip the onion chunks halfway through if they are charring faster than the tomatoes.
Let them cool for 10 minutes to prevent the blender lid from bursting off and potentially burning you. Add the tomatoes and onions to a blender. Use your hands to squeeze the roasted garlic out of the bulb and into the blender.
Purée the veggies with half of the broth and all of the basil leaves until it’s smooth and creamy. You may need to do this in batches depending on the size of your blender. Another option is to use an immersion blender in a pot.
Add the puréed mixture, the rest of the broth, the plant milk, sugar, salt, pepper, and oregano to a medium-sized pot. Simmer the soup on low for 10 minutes to bring the flavors together. Taste it to see if you’d like to add any more salt and pepper. Enjoy!