Combine the cumin, chile powder, salt, pepper, sugar, lime zest and cilantro in a small bowl. Place the salmon flat, skin-side up, in a long baking dish. Make 10 to 12 1-inch slashes in the skin with the tip of a knife, just deep enough to reach the flesh. Rub 1/3 of the salt mixture into skin, then flip the fish over, spoon the tequila over the fish and rub the remaining 2/3 of the salt mixture on the salmon.
Cover the fish tightly with plastic wrap, then place a slightly smaller baking dish directly on top of fish and weigh it down with heavy unopened cans. Refrigerate for 2-3 days, basting with the resulting juices each night and re-covering and weighing back down again.
When ready to serve, remove the gravlax from the refrigerator. Thinly slice the salmon flesh, taking care not to cut through the skin and arrange the pieces on a platter. Serve with lightly toasted flour tortillas or toasted whole grain bread (keep warm in a clean kitchen towel, bread basket or tortillero), and let everyone garnish with the crema, avocado and a squeeze of fresh lime juice.