In a large heavy-bottomed pot over medium heat, melt butter. Stir in onions and cook until softened, about 5 minutes. Stir in flour cook, stirring frequently, until paste begins to darken, 1-2 minutes.
Slowly whisk in chicken broth. Then, stir in crushed tomatoes, baking soda, ½ teaspoon salt, bay leaf, and brown sugar and bring to a boil.
Reduce heat to medium-low and simmer until slightly thickened, about 4-5 minutes. Remove from heat.
Discard bay leaf, and puree soup in batches to desired consistency. Return pureed soup to the pot and stir in cream. Season with salt and pepper, and serve with garnishes (optional).
This soup will serve 6 if you're serving it with something else like salad or grilled cheese.