Combine beef stock, black beans, tomatoes, steak, tomato sauce, onion, green bell pepper, red bell pepper, milk, cumin, fajita seasoning, garlic, lime juice, and red pepper flakes in a large pot over medium heat. Cover pot and simmer for 30 minutes. Add rice and cook until rice is tender, about 30 minutes.
Garnish soup with Monterey Jack cheese, avocado, and tortilla chips.
Per Serving: 488 calories; protein 29.4g; carbohydrates 44.1g; fat 22.4g; cholesterol 57.4mg; sodium 1201.5mg.