How to Make It
Preheat grill to medium (about 400°F). Stir together ground chipotle chiles, brown sugar, coriander, garlic powder, onion powder, pepper, and 1 1/2 teaspoons of the salt in a small bowl. Brush both sides of steak with olive oil; rub spice mixture over both sides of steak, pressing to adhere.
Grill steak, covered with grill lid, to desired degree of doneness, about 7 minutes on each side. Remove from grill, and let rest 10 minutes. Cut diagonally across the grain into thin slices.
Mash avocado, and stir together with lime juice and remaining 1/2 teaspoon salt; spread over tostadas. Top with steak slices and desired toppings. Serve with lime wedges.