Spinach Frittata with Green Salad

INGREDIENTS

• 1 large egg plus 2 large egg whites
• 2 tablespoons grated aged cheese, such as Gruyere
• 2 teaspoons whole milk
• Coarse salt and ground pepper
• 1 teaspoon extra-virgin olive oil
• 1/2 small shallot, minced
• 1 1/2 cups baby spinach
• Nonstick cooking spray
• 2 cups salad greens
• 1/4 teaspoon cider, sherry, or red-wine vinegar