Preheat the oven to 425°F. Line a baking sheet with aluminum foil and set aside.
Arrange the tomatoes, cut side up, on the baking sheet and drizzle with 2 tablespoons of olive oil. Season to taste with salt and pepper. Roast until softened and beginning to brown, 35 – 40 minutes. Cool slightly.
In a large heavy bottomed pot, heat the remaining 2 tablespoons of olive oil over medium-high heat. Add the red onion and saute, stirring occasionally, until translucent, 3 – 4 minutes. Add the garlic and saute until fragrant, about a minute. Add the roasted tomatoes, canned crushed tomatoes, 1/4 cup of basil, a pinch of red pepper flakes (if using) and 3 cups water. Bring to a boil, then reduce heat to low and simmer, partially covered, until thickened, about 30 minutes. Stir in the remaining 1/4 cup basil.
Use an immersion blender to blend the soup in the pot, or, carefully transfer to a blender to puree. If going this route, make sure to cover the blender with a dish towel so the splatter does not spray up and burn you! Add water if you would like a thinner soup, then return to the stove and stir in 1/4 cup cashew cream, plus more if desired. Season to taste with salt and pepper, and enjoy with your favorite toasted gluten free bread or avocado toast!
Keeps for two days in the fridge or 3 months in the freezer.