Preheat oven to 425°F. Line a sheet pan with parchment paper.
Wash and cut the tomatoes (cut cherry tomatoes in half and larger tomatoes in quarters or eights).
Place the onion and tomatoes on the prepared sheet pan. Drizzle oil over tomatoes and onion and add salt and pepper. Toss to coat.
Place the sheet pan in the oven and bake for 20 minutes.
Add the smashed garlic cloves to the pan and lightly toss in the liquid that has collected on the sheet pan. Place the sheet pan back into the oven and continue to cook for 10 minutes.
In a high speed blender, add ¼ cup of the broth. Add the roasted tomatoes, garlic, and onions.
Blend slowly until the tomatoes have broken down before blending at high speed. Blend until the mixture is smooth and there are no more chunks left.
Transfer tomato mixture to a pot and add the broth. Mix until fully combine. Add the basil, red pepper, thyme, and milk. Stir and bring the soup to a simmer.
Simmer for 10-15 minutes. Season to taste with salt and pepper and serve.