Preheat oven to 350 degrees F. Lightly spray 9x13" dish with cooking spray.
In large skillet, heat 1 tbsp of olive oil over medium-high heat.
Add beef and cook 5-7 minutes, stirring occasionally to turn, until it is cooked through.
Remove from skillet and set aside on a plate.
Add additional 1-2 tbsp of oil to skillet and add peppers and onions.
Cook until tender and maybe a little charred on some sides, if that's how you like your fajita vegetables. This will take about 10-12 minutes.
While the veggies are cooking, mix together your fajita seasoning spices in a small bowl.
Once veggies are cooked, add beef back to skillet, then add fajita seasoning and stir well to combine. Remove from heat and get ready to assemble enchiladas.
Spread a thin layer of enchilada sauce on the bottom of the baking dish, about 1/4 cup.
Fill each tortilla with ~1/4 cup of filling and 2 tbsp of cheese (I just estimated - I did not measure for each one) then roll up and place into baking dish, seam side down. Try to save about 1/2 cup of the veggies for topping.
Pour the remaining enchilada sauce over the filled tortillas, then top with remaining cheese.
Bake in preheated oven for 20-25 minutes, added the remaining veggies on the top along the middle for the last 5 minutes to heat them back up, until cheese is melted and enchiladas are bubbling.
Top with chopped cilantro and avocado. Enjoy!