In a pot or dutch oven over medium-low heat, melt butter and add chopped onions. Sauté until the onions start to turn translucent.

Add minced garlic and sauté for 2 more minutes, making sure that the garlic doesn't start to burn.

To the garlic-onion mixture, add a can of diced tomatoes, juice and all.

Add salt, basil, and broth. Simmer for 20-30 minutes.

Puree with an immersion blender, or place in a blender and blend in batches. (Fill blender pitcher halfway and cover with a towel.)

If you used a traditional blender, pour the blended soup back into the pot and mix well.

Ladle the soup into bowls and top with cheese, croutons (omit if gluten free), scallions or a dollop of sour cream.