The Ultimate Two-Beef Chili

Name: The Ultimate Two-Beef Chili
Prep time: 1 h
Total time: 1 h
Cuisine: American, TexMex
Ingredients: 2 tbsp. olive oil; 3 lb. chuck, cut into 1-inch pieces; Kosher salt and freshly ground black pepper; 1 yellow onion, chopped; 2 lb. ground beef; 8 cloves garlic, minced; 2 tbsp. chili powder; 2 tbsp. ground cumin; 2 tsp. red pepper flakes; 2 c. beef broth; 2 (28-ounce) cans diced tomatoes; Chopped scallions, grated Cheddar, sour cream, and diced avocado, for serving