Lindsey's Favorite Shrimp Ceviche Cocktail   

Ingredients:

• 1/4 cup chopped red onion
• 2 small limes, squeezed
• 1 tsp olive oil
• 1 lb large cooked shrimp, peeled and deveined*
• 1 medium hass avocado, diced into chunks
• 1 medium tomato, diced
• 1 cup diced English cucumber, not peeled
• 1 serrano pepper, seeds removed and minced
• 2 tbsp chopped cilantro, plus more for garnish
• salt and fresh black pepper to taste
• 1 lime cut into wedges for serving
• 2 1/4 cups shredded iceberg lettuce