In a large bowl, toss onion with lime juice and ½ tablespoon oil and let sit for 5 minutes.
Heat ½ tablespoon oil in a large skillet on medium high. Season shrimp with ¼ teaspoon each salt and pepper and cook until opaque throughout, 2 to 3 minutes per side.
Toss tomatoes with onions, then toss with lettuce and cilantro. Divide among bowls and top with shrimp, avocado and egg.
