Combine lettuce, avocado, mango, blueberries, sun-dried tomatoes, and onion in a large bowl.
Heat olive oil and butter in a skillet over high heat until starting to brown and lightly smoking. Add shrimp. Cook until shrimp are lightly browned and cooked through, rotating often, 3 to 5 minutes. Add lemon juice and pepper at the last minute. Cook until most of the liquid is gone, about 2 minutes more. Remove from heat and let cool slightly.
Add enough dressing to the lettuce mixture to moisten the ingredients. Plate salad and add shrimp on top.