Red Grapefruit and Fennel Salad

Name: Red Grapefruit and Fennel Salad
Ingredients: 3 ruby-red grapefruit; 1/4 teaspoon whole fennel seeds; 1 teaspoon honey; 1/2 teaspoon Dijon mustard; Salt and freshly ground black pepper; 1 tablespoon extra-virgin olive oil; 1 small red onion; 1 small fennel bulb; 8 ounces baby spinach or watercress