Heat 2 tablespoons olive oil in a large pan over medium-low heat, add the onions, peppers, and garlic and cook for 15 minutes or until they start to caramelize, stirring frequently. Add a little water to speed up the process. Season with salt and pepper. Set aside.
Liberally season the steak with salt and pepper.
In the same pan, increase heat to medium-high, sear the steak slices for about 2 minutes per side, until the internal temperature reaches medium-well. Set aside to rest then dice steak into bite-sized pieces.
To assembe, lay a tortilla flat and layer with shredded cheese, steak, and fajita veggies on one half of the tortilla, fold in half.
Wipe the skillet clean. Over low heat, add 1/2 tablespoon of butter. Cook the quesadilla until golden brown on the bottom. Flip and repeat, cooking until cheese is completely melted through.
While the quesadilla is cooking mix together the chipotle crema. In a small bowl, combine mayo, sour cream, chipotle sauce, lime juice, and a pinch of salt. Mix well and serve alongside the quesadillas.