Greek Salad with Fennel

Name: Greek Salad with Fennel
Ingredients: 1/2 small red onion; 1 tablespoon red wine vinegar; Juice of 1 small lemon; 1 teaspoon salt, plus more to taste; 1/4 cup olive oil; Freshly ground black pepper; 1/4 pound Greek feta; 1/2 large fennel bulb, with fronds; 6 cups roughly chopped Romaine (2/3 small head); 1/4 cup roughly chopped roasted red pepper; 1/4 cup green olives, sliced; 1/4 cup Kalamata olives, whole