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 (I love my garlic press to mince the garlic fast, but you can also just slice it, since it will all be thrown into the blender at the end) 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. I would recommend only filling your blender pitcher up about half way, and make sure to place a towel over the lid. Just in case any of the soup splashes up, you don't want it burning your hand.
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 you are gluten free), scallions or a dollop of sour cream to make it a little creamy and tangy.