For the skirt steak: Combine the chili powder, salt, cumin, chipotle powder, cayenne and lime zest and in a small bowl. Rub the spice mixture over entire skirt steak and let marinate in a zip-top bag at least 20 to 25 minutes.
Preheat a grill pan or cast-iron skillet over high heat. Preheat the oven to 425 degrees F.
Grill the steak 3 minutes per side for medium-rare. Let rest 5 minutes before slicing thin against the grain.
For the quesadillas: Place 6 tortillas on a sheet tray, brushing the outsides with oil. Sprinkle 1/4 cup of the shredded cheese on each tortilla along with 2 tablespoons of the Tomatillo Guacamole and 1/6 of the sliced skirt steak. Top with the remaining tortillas and brush with oil.
Bake the quesadillas 15 to 20 minutes, carefully flipping halfway through cooking. The cheese should be melted and the tortillas should be crisped. Transfer to a cutting board or serving platter. Cut into wedges and serve with sour cream, cilantro, lime wedges and remaining Tomatillo Guacamole.
Combine the cilantro, cumin, salt, tomatillos, garlic, avocado, lime juice and onions in a food processor or blender and pulse until combined.