Preheat the oven to 400℉.
Add the onion, tomatoes, and garlic to a rimmed baking sheet. Drizzle with oil, then sprinkle with salt and pepper. Toss to coat and spread in an even layer.
Bake for about 30 minutes, stirring halfway through, until the tomatoes have shriveled.
Remove from the oven and cool for 5 minutes.
Add the vegetable mixture, broth, tomato paste, oregano, and non-dairy milk to a blender. Process until smooth, then transfer to a saucepan.
Heat the soup over medium-low heat, stirring occasionally. Cook for about 10 minutes, or until warmed through. Taste and season with additional salt and pepper as needed.
Top with fresh basil and serve!