Orange slices are a common addition to spinach salad, but grapefruit is also delicious.
With a sharp knife, peel the grapefruit, removing all the bitter white pith. Slice about 3/8 inch thick.
In a large bowl, whisk together the vinegar, mustard, and 1/8 teaspoon each salt and pepper. Drizzle in 1/2 cup of the oil, whisking to combine.
Season the shrimp with 1/2 teaspoon salt and 1/8 teaspoon pepper. In a large nonstick skillet, heat the remaining tablespoon oil over medium heat. Add the shrimp, and cook, turning once, until pink, about 3 minutes per side.
Add the grapefruit, cooked shrimp, spinach, and onion to the vinaigrette in the large bowl. Toss well. Serve at once.