2 cups mixed shredded cheese (Mexican blend works great)
In a bowl, mix together garlic, chili powder, cumin and butter to form a paste. Set aside.
Season flank steak with salt and pepper and heat a grill pan or cast-iron pan over high heat. Sear flank steak until golden on one side (about 3 minutes).
Flip over steak, turn down heat to medium and smear both sides with the butter mix, cooking until you've reached desired doneness. Remove from heat, and allow to rest slightly. Slice on a bias into 1/2-inch bites.
To assemble, layer a tortilla with sour cream, chiles, 1 tostada shell, steak, cheese and hot sauce. Bring sides of the tortilla to the middle forming a hexagon (stop sign) shape. Continue with remaining quesadillas.
Heat your pan over medium heat and sear quesadillas, seam side down, until golden on both sides and cheese is melted.