- 3 pink grapefruit
- 3 tablespoons wine vinegar
- 2 teaspoons Dijon mustard
- Salt and pepper
- 1/2 cup plus 1 tablespoon vegetable oil
- 1 1/2 pounds large shrimp, peeled and deveined
- 1 1/2 pounds spinach, stems trimmed and leaves washed well (about 3 1/2 quarts)
- 1/2 red onion, thinly sliced