Green Herb Ceviche with Cucumber

Name: Green Herb Ceviche with Cucumber
Ingredients: 1/2 head garlic, cloves broken apart; 2 or 3 fresh serrano chiles; 1 medium bunch cilantro, thick bottom stems cut off (1 cup packed); 1 small bunch flat-leaf parsley, thick bottom stems cut off (1 cup packed); 1/2 olive oil; Salt; 1/4 fresh lime juice, plus more if needed; 1 1/2 sashimi-quality skinless, boneless fish fillets, such as Alaskan halibut or ahi tuna, cut into 1/2-inch cubes; 7 small pickling cucumbers or Persian (baby) cucumbers, cut into 1/2-inch cubes; Salt, if needed; 2 ripe large avocados, pitted, flesh scooped from skin and cut into cubes; Lettuce leaves, preferably butter lettuce, for garnish