Heat oil in a Dutch oven over medium-high. Add onion; sauté until tender, 5 to 7 minutes. Stir in Italian seasonings; cook until fragrant, less than a minute. Add tomatoes and baking soda, then chicken broth; bring to a simmer. Reduce heat to low and simmer, partially covered, to blend flavors, about 10 minutes.
Working in batches, pour soup into a blender; puree, making sure to vent blender by removing pop-out center, and draping a kitchen towel over the top, until creamy smooth, 30 seconds to a minute. Return soup to pot, stir in evaporated milk, and heat to simmer, seasoning with salt and pepper to taste. (Can be refrigerated up to 3 days; return to simmer.) Ladle into bowls and serve.