Chile con Queso

INGREDIENTS

• 2 tablespoons unsalted butter
• ½ cup chopped yellow onion
• 2 serrano chiles, seeds and stems removed, diced
• 1 jalapeño pepper, seeds and stems removed, diced
• 2 cloves garlic, minced
• 2 tablespoons all-purpose flour
• 1 cup whole milk
• ½ cup chopped cilantro
• 1 (15-ounce) can diced tomatoes, drained (about 1 cup)
• 12 ounces cheddar cheese, grated (about 3 cups)
• 12 ounces Monterey Jack cheese, grated (about 3 cups)
• ½ cup sour cream