Step 1Cut the shrimp into 1-inch pieces and combine it in a bowl with the onions, salt and lime juice. Cover tightly and refrigerate until the shrimp is firm and white, about 3 hours.
Step 2Cut the radishes into half-inch wedges and add to the ceviche. Cut the corn kernels from the cob and add to the ceviche. Peel and pit the avocado, dice the flesh and add it to the ceviche. Tear the mint leaves into quarters and gently stir everything together.
Step 3Serve in juice or martini glasses or in a bowl with tortilla chips.