In a large pan, melt the butter over medium heat. Add in the diced onion and jalapenos and sauté for 5 minutes, until translucent. Add in the tomatoes and garlic and continue to sauté for another minute.

Sprinkle the flour into the vegetable mixture, and mix together with a whisk. Slowly add milk, stirring constantly after each addition, until all the milk has been incorporated and most of the lumps are gone.

Slowly add in the cheese, 1 cup at a time, mixing and allowing it to melt before adding more cheese. Once all the cheese is melted, add in the sour cream and stir. Add in salt and pepper to your taste.

Serve this yummy dip alongside of chips or cut-up vegetables.