For the egg salad, in a bowl, combine the eggs, cheeses, and onion. Set aside. In a separate bowl, combine the mayonnaise, yogurt, mustard, and pepper; mix well. Add to the egg mixture, and mix until combined. Cover and refrigerate at least 2 hours before using. For the sandwiches, spread about 1/3 cup of the egg salad on 1 slice of bread, and top with 4 slices of cucumber. Top with a slice of cheese and a lettuce leaf. Top with another slice of bread. Cut and serve with your favorite homemade shredded carrot salad, lemon dressing, and sliced fruit. Variations: Substitute smoked Cheddar for regular Cheddar in the salad. Other sliced cheeses may be substituted in the sandwich also. Try substituting chopped egg whites instead of whole eggs. Try low-fat or no-fat mayonnaise, instead of real mayonnaise.