Preheat oven to 425°. Cut tomatoes in quarters, removing the tough piece where the stem attaches.
Add tomatoes to a large baking dish, along with chopped onion and whole garlic cloves. Drizzle with olive oil and season generously with salt and pepper. (Optionally, add red pepper flakes.) Toss to coat, then place in the oven and roast for about 1 hour, or until tomatoes and onions are very soft and have picked up some golden brown color. If needed, add additional cook time 10 minutes at a time.
Transfer roasted tomatoes, onion, and garlic to a pot, along with the juices at the bottom of the pan. Add vegetable broth, fresh basil, optional oregano, and cashew cream (or cream substitute of choice).
Bring to a boil over medium-high heat, then reduce heat to low. Use an immersion blender to purée the soup to your preference. (For a perfectly smooth soup, a traditional blender will work best.)
Safety tip. If using a blender, make sure the pitcher is heat-safe. Exercise caution while blending hot ingredients. Do not fill the blender up up all the way, as pressure can build rapidly. Leave the chute on the lid open, and hold a clean kitchen towel over it while pulsing. Increase speed very gradually. This will allow some of the steam to escape to keep the pressure from building up or your lid from flying off.
Soup is ready to enjoy once blended to your preference.