Make the meat mixture: in a large pan, heat 2 tablespoons of oil over medium heat. add chopped onion and sauté for 4-5 minutes. Add crushed garlic and sauté for 1 minute. Add diced pepper and cook for 4-5 minutes.
Add ground beef and cook, breaking it apart until no longer pink, about 6-7 minutes. Season with salt, pepper, paprika, oregano, cumin and red chili flakes. Stir well. Add crushed tomatoes and cook for 5 minutes over medium heat.
Prepare the toppings: thinly slice the lettuce, in a small bowl mash the avocado with a teaspoon of lemon juice and pinch of salt, chop red onion and prepare the cheddar cheese.
Assembly: Heat taco shells as directed on package. Fill taco shells with 2-3 tablespoons of meat mixture, top with avocado, red onion, lettuce and cheese.