Using a five to six quart stockpot, over medium-high heat, add the olive oil and butter.: The moment the fats hit the warm pot you will hear a soft sizzle and smell the buttery sweetness combine with the peppery fruit of the extra virgin olive oil . This sound signals the pan is ready for the onion . The purpose of heating the fats first is to create an even coating for the flour later, which helps the roux develop smoothly. Be careful not to let the butter brown too quickly, as that will add a nutty, rather than sweet, note and change the profile. If the fats start to smoke, lower the heat immediately and wipe the pan if necessary.
Add the finely diced onion and fresh minced garlic. Cook for five to seven minutes, stirring often until the onion is translucent.: At first you will hear gentle bubbling and see small translucent edges appear on the onion . As they cook, a sweet fragrance will emerge and the kitchen will feel warmer. Stirring often ensures even softening and prevents the garlic from sticking and burning, which would impart bitterness. A common mistake is rushing this step with higher heat, which yields browned edges and a sharper, less mellow flavor, so keep the heat moderate and patient.
Whisk in the flour and continue to cook and stir for two minutes.: After the onion is translucent, adding the all purpose flour and whisking creates a light roux that will thicken the soup. You will notice the flour absorb the fats and form a paste with a faint toasted smell as the raw flour cooks out. This step matters because it prevents a grainy texture later and gives the soup body without cream. Do not skip cooking the flour long enough, or the raw, chalky taste will remain.
Add the tomato puree, vegetable broth, and tomato paste. Stir to completely incorporate the tomato paste.: Pour the deep red tomato puree and the vegetable broth in slowly while whisking so the roux disperses evenly. When you stir in the tomato paste it should dissolve and darken the liquid, creating a cohesive color and scent that turns richer and more savory. This melding is crucial for uniform flavor. If you add cold liquid too quickly the roux may clump, so temper gradually and keep whisking to avoid lumps.
Add the granulated sugar, kosher salt, dried basil, cracked pepper, red pepper flakes, and whole bay leaves. Stir occasionally until the soup begins to simmer. Lower the heat to medium and continue to cook for 15 to 20 minutes, stirring occasionally.: Once the aromatics and liquids combine, you will notice steam rising and the soup giving off a deep tomato aroma. The granulated sugar will subtly round the acid, while the dried basil and spices release their oils into the hot liquid. As it reaches a simmer, gentle bubbles should break the surface rather than a rolling boil, which keeps the texture smooth. Simmering allows flavors to meld and the soup to thicken naturally. A frequent error is boiling too vigorously, which can separate flavors and make the texture thinner, so keep it at a steady simmer and stir occasionally to prevent sticking.
Remove the soup from the heat. Remove the bay leaves before serving.: When you take the pot off the stove, the aroma mellows slightly and the surface calms. Removing the whole bay leaves is essential because they will continue to infuse bitter, astringent notes if left in the serving bowl. The rest period after heat helps the flavors settle and gives you a chance to taste and adjust seasoning. Avoid serving with bay leaves still in the pot, as someone may bite into one, which is unpleasant.
Garnish with fresh, thinly sliced basil.: The final visual cue is the pop of green from the thin basil slices floating on top, releasing a fresh, herbal perfume as soon as the bowl is set down. Garnish just before serving so the leaves remain bright and do not wilt into the hot liquid. This touch ties the whole dish together with freshness. One small pitfall is adding the garnish too early, which will cause the basil to lose its color and aromatic impact.