Heat oven to 450 degrees F. Cover a large baking sheet with foil. Lay tortillas on sheet and spray with cooking spray. Turn over and spray other side. When oven is heated, bake for 10 to 15 minutes or until light golden brown. They may still appear floppy, but will become crispy as they cool. Remove and place on a plate covered with paper towels. Blot with a paper towel to absorb any excess oil.

In a small bowl, combine the cumin, garlic powder, pepper, salt, chili powder, brown sugar, and oregano. Put the olive oil in a Ziploc bag and mix in all but 1 tablespoon of the spice mix. Add the garlic and shrimp; massage to coat. Set aside.

Heat a 12-inch sauté pan over medium-high heat, and, when hot, coat with cooking spray. Add the sliced onion and sauté until softened and fragrant and starting to brown, about 4 minutes. Add the chile and bell pepper and continue to sauté for 2 minutes. Add the remaining spice mixture and sauté until the onion and chiles are tender but not mushy, another 1 to 2 minutes. Transfer to a bowl. Return the skillet to medium high heat and add the shrimp. Cook, stirring frequently, until the shrimp are cooked through, 2 to 3 minutes. Add the sautéed vegetables; toss to combine and reheat.

Spread a small amount of guacamole on tostada. Spread a small amount of sour cream over guacamole, then add some lettuce. Add some sauteed vegetables and top with shrimp. Sprinkle with chopped cilantro.

Serve with additional sour cream and guacamole, and a wedge of lime.