
In a medium saucepan over medium heat, add the butter and onions. Cook until the onions soften, about 5 minutes. Add the garlic and sauté until fragrant.
Add the crushed tomatoes, vegetable broth, salt, and pepper, and bring the mixture to a simmer. Continue to simmer uncovered, for 40 minutes. Add the fresh basil, then, use an immersion blender to blend the soup until smooth and consistent.
Ladle the tomato soup into bowls and serve with more fresh basil, if desired.