Prepare a grill for medium-high heat; lightly oil the grates.
Put the garlic, jalapeno and onion in a food processor and pulse until finely chopped. Add the tomatillos, avocado and 1/2 teaspoon salt and pulse until chopped but still chunky. Transfer to a bowl, stir in the chopped cilantro and adjust the seasoning with additional salt; set aside.
Toss the shrimp with the oil and season generously with salt. Grill, turning once, until just cooked through, about 2 minutes per side.
Warm the beans in a small skillet over medium heat. Spread a thin layer on each tostada and top with some lettuce and a spoonful of the avocado-tomatillo salsa. Top with 3 to 4 shrimp per tostada and sprinkle with some cilantro leaves.
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