Preheat oven to 375°.
In a large skillet over medium-high heat, brown the ground beef, onion and garlic. Break up meat using a wooden spoon and season with salt and pepper. Sauté about 5 minutes; add bell pepper and cook 2 to 3 minutes more.
Add tomatoes and chipotle; bring to a boil and simmer until warmed through, 2 to 3 minutes. Stir in cilantro and top with cheese. Bake until cheese is melted, 3 to 4 minutes.
Garnish with avocado and more cilantro. Serve with sour cream.
