Toss the tomatoes, onion and garlic with the olive oil and a pinch of salt and pepper.
Roast the tomatoes, onion and garlic on a parchment-lined sheet pan for 40-50 minutes on 400°f or until the tomatoes start to shrivel and the onions and garlic are slightly browned.
Put the roasted vegetables in a medium pot over medium heat and add in the milk.
Cook the soup only until the milk is hot then add in the basil and purée.
Taste the soup and season with salt, pepper, and sugar as needed.