Cut the tomatoes in half lengthwise. Peel the onion and cut it into quarters. Peel the garlic cloves.
Place the tomatoes, onions, and garlic on a baking sheet. Drizzle olive oil over them. Season with salt and pepper, then mix everything together until well combined.
Bake in the preheated oven for 40 minutes. Remove the tray from the oven and let it cool for 10 minutes.
Transfer everything from the baking sheet, including the accumulated juices, to a blender. Add the tomato paste to the blender and blend until smooth.
Stack the basil leaves on top of each other and slice them into thin strips. Divide the soup evenly into four bowls and garnish each portion with the basil strips. Serve warm.
Notes
If your tomatoes are more acidic, add a pinch of sugar or a drizzle of maple syrup to balance the flavor. This is especially helpful if you’re using canned tomatoes.
Fresh basil is best for this recipe. If you don't have fresh, dried basil can be used, but the flavor will be less intense.