In “The Essentials of Classic Italian Cooking,” Marcella Hazan shared her now-famous recipe for a sublimely delicious, remarkably simple sauce made with nothing more than tomatoes, onion and butter. This soup turns that sauce into a light meal. If you own an immersion blender, the soup can be pureed directly in the pot, but the texture won’t be as velvety as if processed in a conventional blender. We top individual bowlfuls with olive oil-toasted baguette slices, a dollop of pesto and shaved Parmesan. Homemade pesto is best, of course, but store-bought is fine; we’ve found, however, that refrigerated pesto tastes fresher than the shelf-stable type sold in jars. A Y-style vegetable peeler is the best tool for planing off wide, thin shavings from a chunk of Parmesan cheese.