Roast tomatillos under hot broiler for 5 minutes, turn over and roast for another 5 minutes. In a blender or food processor, puree tomatillos, jalapeno, garlic, cilantro, and avocado until coarsely puréed. Pour into a bowl. Add lime juice and salt to taste. Set aside.

In a small pan, heat 1/4 olive oil and fry corn tortillas over medium-high heat. Sprinkle with salt and allow to drain on paper towels.

In a small saucepan, heat up canned black beans, drain, and rinse with water. In a medium pan, heat 2 tsp olive oil. Cook shrimp with juice from one lime, garlic, salt, and pepper. Once cooked, mix in chopped cilantro.

To build tostadas, start with a layer of salsa, followed by a layer of beans and shrimp, and topped with a squeeze of lime juice and fresh cilantro, if desired.