Heat oil in a large saucepan over medium heat until it is shimmering. Add onions, pepper, chili powder, cumin, cayenne, and ½ tsp salt. Cook until vegetables have softened, about 7 minutes. Stir in garlic and cook for another 30 seconds.

Add ground beef, increase heat to medium-high. Cook until no longer pink, about 10 minutes. Stir in beans, diced tomatoes (with juice), tomato puree, and remaining ½ tsp salt. Bring to a simmer, then reduce heat to maintains a slow simmer, cover and cook 45 minutes. Remove lid and continue to simmer an additional 45 minutes. Season with additional salt and pepper to taste.

Preheat oven to 375F. Transfer chili to a baking dish. Top with Fritos, pressing them lightly into the chili. Sprinkle Cheddar cheese over top and bake until cheese is melted and chili is bubbling, about 30 minutes. Sprinkle with sliced green onions and serve immediately.