Preheat oven to 350°. Put tomatoes, garlic, half of the jalapeño, 1/4 cup cilantro, and the lime juice in a food processor and pulse a few times to chop. Add salt and pepper to taste. Set salsa aside.
In a large skillet, heat oil over medium heat. Sauté onion and remaining jalapeño until soft, 4 minutes. Add crab and cook just until crab is warm, about 2 minutes.
Put taco shells or tortillas on a baking pan and warm in oven, about 3 minutes. Arrange on a platter and fill with crab mixture, dividing evenly. Top crab with lettuce, cheese, and avocado. Sprinkle tacos with green onion and remaining 1/4 cup cilantro. Serve immediately, with salsa on the side or on top.
Note: Nutritional analysis is per taco.