Preheat grill to 350° to 400° (medium-high) heat. Grate zest from lime to equal 1 tsp. Cut lime in half, and squeeze juice from lime to equal 1 Tbsp.
Sauté onion and garlic in hot oil in a medium skillet 1 minute. Add corn, and cook 2 to 3 minutes or just until corn begins to brown. Transfer mixture to a medium bowl. Stir in lime zest, lime juice, avocado, cilantro, and 1/2 tsp. salt.
Combine turkey, next 4 ingredients, and remaining 1 tsp. salt. Gently shape mixture into 6 (4-inch) patties.
Grill patties, covered with grill lid, 4 to 5 minutes on each side or until done. Serve on tostadas with corn mixture and desired toppings.