Heat the olive oil in a large stainless steel stockpot or enameled Dutch oven over medium heat. Add the onions and cook, stirring occasionally, until the onions soften and begin to turn translucent, about 5 minutes.

Add the garlic and cook, stirring, for 1 minute. Add the tomato paste and dried basil and cook, stirring, for 2–3 minutes.

Add tomatoes (and their juices), stock, sugar, salt, and pepper. Bring to a boil, then reduce heat to medium-low and simmer for 30 minutes, stirring occasionally. If you’d like a thicker soup, let it simmer for a little longer.

Remove from heat. Using an immersion blender, puree the soup until smooth. Or let cool slightly and then carefully transfer the soup to a regular blender and blend in batches (see Notes for instructions).

Stir or blend in the heavy cream. Taste. Add in crushed red pepper flakes, if desired. Serve warm.

Keep leftovers in a covered container in the refrigerator for up to 1 week. To reheat, simply pour into a pot and cook over medium heat until warm.