Prepare the marinade: Whisk garlic, adobo sauce, olive oil, lime juice, chili powder, cumin, oregano, smoked paprika, and salt in a ziptop bag.
Marinate steak: Place steak into the bag, seal, and refrigerate for at least 30 minutes or up to 24 hours.
Prepare vegetables: Heat 1 tablespoon of olive oil in a cast iron skillet over medium-high heat. Add sliced bell pepper and onion, sauté until lightly charred but still crunchy (about 3-4 minutes). Season with salt. Remove vegetables from skillet and wipe clean.
Cook steak: Remove steak from the fridge about 30 minutes before cooking. Add a tablespoon of olive oil or avocado oil to the skillet over medium-high heat. Sear steak for about 3 minutes per side, or until a nice crust forms and the internal temperature reaches 125-135°F (52-57°C) for medium-rare to medium. Transfer steak to a cutting board and rest for 5 minutes. Cut into small cubes.
Assemble quesadillas: Wipe the skillet again and place over medium heat. Lay one tortilla flat in the skillet, add a third of the sautéed vegetables on one side, then a third of the steak cubes, and sprinkle with ¼ cup cheese. Fold tortilla in half over the filling.
Cook quesadillas: Cook until the tortilla is crisp and cheese has melted, about 2 minutes per side. Repeat with the other tortillas and remaining ingredients. Remove quesadillas, cut each into two pieces, and serve warm with sour cream.