How to Make It
Peel and section grapefruit over a bowl; squeeze membranes over a bowl to extract juice.
Combine lemon juice, honey, mustard, salt, pepper, and 3 tablespoons reserved grapefruit juice in a bowl, stirring with a whisk. Gradually add oil, stirring constantly with a whisk. Place 1/4 cup vinaigrette in a bowl; add endive. Let stand 10 minutes.
Arrange half of arugula on a serving platter, and top with endive mixture, remaining half of arugula, walnuts, cheese, and reserved grapefruit sections. Drizzle with remaining vinaigrette.