Heat the olive oil in a large pot over medium heat. Add the diced onions and cook for 5-7 minutes, stirring occasionally, until they become soft and translucent. Once the onions are tender, add the tomato paste and sea salt, then stir constantly for 4-5 minutes until the tomato paste deepens in color and caramelizes slightly—this concentrated cooking step is crucial for developing rich, complex flavor. I like to let the tomato paste stick to the bottom of the pot just a bit before stirring; it creates deeper flavor compounds through the browning process.

Add the minced garlic and fresh rosemary to the pot and stir continuously for about 1 minute, allowing the garlic to become fragrant but not browned. This brief cooking time releases the aromatic oils from both ingredients and distributes their flavors throughout the base without allowing them to burn, which would create bitterness.

Pour the whole peeled tomatoes along with their juice and the vegetable broth into the pot, stirring to combine everything. Break up the larger tomato pieces with the back of a wooden spoon as you stir. Sprinkle the baking soda over the mixture—you'll see it fizz slightly, which is exactly what you want; this chemical reaction neutralizes the acidity of the tomatoes and gives the final soup a smoother, more balanced taste rather than sharp or tangy.

Bring the soup to a simmer over medium heat, then reduce heat to low, cover the pot partially with a lid, and let it simmer gently for 20 minutes. This extended cooking time allows all the flavors to meld together and the tomatoes to break down further, creating a more cohesive and developed flavor profile. I find that partially covering the pot (leaving a small gap) prevents the soup from becoming too reduced while still allowing gentle evaporation.

Remove the pot from heat and let it cool for 1-2 minutes. Using an immersion blender, puree the soup until it reaches your desired consistency—I prefer a completely smooth soup, so I blend until there are no visible tomato chunks remaining. If you prefer a chunkier texture, blend for less time. Work the blender in overlapping motions throughout the pot to ensure even blending.

Stir in the heavy cream and lemon juice, then taste the soup and adjust seasoning with additional salt and black pepper as needed. The lemon juice brightens the flavors and adds a subtle complexity that balances the richness of the cream without making the soup taste citrusy. Stir well to fully incorporate the cream and create a uniform, silky texture.

Ladle the hot soup into bowls and serve immediately. If desired, garnish with a drizzle of cream, fresh rosemary, croutons, or a grilled cheese sandwich on the side.