Preheat oven to 375°F.
In a large skillet over medium heat, cook ground beef until browned. Drain excess grease.
Add diced onion, bell pepper, and garlic. Sauté until soft, about 5 minutes.
Stir in taco seasoning, salt, pepper, and tomato sauce. Let it simmer for 3–5 minutes.
Remove from heat and stir in sour cream.
Pour mixture into a greased 8x8 casserole dish.
Sprinkle shredded cheese over the top, and add olives if using.
Bake uncovered for 20 minutes until cheese is melted and bubbly.
Top with chopped green onions and sliced avocado before serving.