1 pound tomatoes, cored and cut into 1/2-inch pieces
6 scallions, white and green parts separated, sliced thin
1/4 cup minced fresh cilantro
3 garlic cloves, minced
2 teaspoons minced canned chipotle chiles in adobo sauce
Salt and pepper
1 1/2 pounds peeled and deveined extra-large shrimp (21 to 25 per pound), tails removed
2 tablespoons vegetable oil
1 tablespoon lime juice, plus lime wedges for serving
1 avocado, halved, pitted, and cut into 1/2-inch pieces