Prep. Let the steak come to room temperature before cooking. Pat dry the steak with paper towels.
Season and sear the steak. Heat oil in a large skillet (or a cast iron skillet) over high heat. You should see smoke. Just before cooking, season the steak with salt and pepper to taste. Sear the steak for 2 minutes, then flip and sear for another 2 minutes. Push the steak aside and add butter.
Baste the steak. Once the butter is melted, spoon it over the steak and cook for 2 minutes or until it reaches 125ºF for medium rare. For medium it should go to 135ºF, and for well-done 145ºF.
Let it rest. Remove the steak from the skillet and allow it to rest for 5 minutes before slicing.
Make the filling:
Slice the veggies. Cut the bell pepper in half and remove the seeds. Continue to cut it into strips. Set aside. Peel and slice the shallots.
Sauté. Heat 1 teaspoon of olive oil in a large skillet over medium-high heat. Add bell pepper and shallots and cook them, without moving them around, for 6-8 minutes or until they just catch some color.
Add seasoning. Season the veggies with salt, pepper, chili powder, cumin, oregano, and cayenne pepper.
Combine and cook. Add the steak to the vegetables and cook until the steak is heated through. Transfer to a plate.
Prepare the tortillas and assemble:
Cook the tortillas. Heat ½ teaspoon of olive oil in a large skillet over medium-high heat. Add the tortilla and swirl it around the skillet so it is coated nicely.
Add cheese. Add one slice of cheddar cheese and ¼ cup of pepper jack cheese over the tortilla.
Add the filling. Add the prepared steak and veggies over the cheese.
Cook. Close the filling with the second tortilla and cook, slightly pressing with a spatula, for 3-5 minutes or until the tortilla is nicely browned. Carefully flip the tortilla and cook on the other side for the same amount of time. Repeat the process with the remaining ingredients.
Slice and serve. Slice the steak quesadillas into quarters. Serve immediately.
Notes
Room temp steak. Always bring your steak to room temperature before cooking. This will ensure your steak is cooked perfectly.
Seasoning. Season the steak just before cooking. Leaving salt for a prolonged time on the steak will just extract the moisture from the steak, leaving it dry after cooking.
Keep them warm. To keep the quesadillas warm during the cooking process, you can preheat your oven to 350ºF. Add in the cooked quesadilla and turn the oven off. This will keep it warm while you are making the second one.
How to flip them. Flipping quesadillas is a bit challenging. I usually place a large plate over the quesadilla, then flip the entire skillet. Afterward, I just slide the quesadilla into the skillet.
Make it less spicy. If you want a milder quesadilla, skip the cayenne pepper and also use mozzarella cheese instead of pepper jack.
Onion options. Shallots can be substituted with onions if you want to.