Pacific Cod Ceviche

Name: Pacific Cod Ceviche
Ingredients: 1 1/2 pounds very fresh Pacific cod; 1 1/2 to 2 cups fresh lime juice; 1 red or white onion, sliced, soaked for 5 minutes in cold water, then drained and rinsed; 2 large garlic cloves, minced; 2 jalapeño or Serrano chilies, minced seeds and membranes removed for a milder ceviche; Salt; freshly ground pepper; 1 ripe but firm Hass avocado, diced; 3 tablespoons extra virgin olive oil; 1/2 cup chopped cilantro (1 bunch); 5 radishes, sliced; 1 head Bibb lettuce or 5- or 6-ounce bag baby spinach for serving