Place the tomatoes (with juices) in a blender; cover and blend until pureed.
In a large saucepan, heat butter over medium-high heat.
Add onion and cook until tender.
Add garlic; cook and stir 1 minute longer.
Remove from heat; stir in the flour until smooth.
Gradually whisk in broth.
Add pureed tomatoes; bring to a boil over medium heat, stirring occasionally.
Reduce heat and simmer for 20-25 minutes to allow the flavors to blend.