Place the pecans in a large skillet, and cook over medium heat 3 minutes or until lightly browned, shaking skillet frequently. Set aside.
Place 2 cups spinach on each of 4 serving plates. Arrange 1/2 cup grapefruit and 1/2 cup mushrooms over spinach. Sprinkle each serving with 1 tablespoon cheese and 1 1/2 teaspoons pecans; drizzle each with 2 tablespoons vinaigrette.