1. In a small bowl, combine olive oil, minced garlic, ground cumin, chili powder, salt, and black pepper. Rub this mixture all over the flank steak, ensuring it is well-coated.
2. Let the steak marinate for at least 30 minutes at room temperature. Meanwhile, preheat your grill to medium-high heat.
3. Grill the steak for about 4-5 minutes on each side, or until it reaches your desired level of doneness. I like it medium-rare, just enough to retain its juiciness.
4. Remove the steak from the grill and let it rest for at least 5 minutes before slicing it thinly against the grain.
5. Warm the refried beans in a saucepan over low heat, stirring occasionally.
6. To assemble the tostadas, spread a layer of refried beans on each tostada shell, followed by slices of the grilled steak.
7. Top with shredded lettuce, pico de gallo, crumbled queso fresco, and avocado slices.
8. Serve immediately with lime wedges for squeezing over the top.

Cooking tip: To get the best flavor, always slice your steak against the grain. This technique keeps the meat tender and easy to chew, a trick I learned from watching my grandmother effortlessly slice through her creations.