1. In a small bowl, stir together the yogurt, dill, mustard, oil, vinegar, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
2. Spread a tablespoon of the dill sauce on one side of each of four slices of bread. Top each slice with lettuce, provolone, tomatoes, a sprinkling of salt and pepper, the cucumber, onion, avocado, and sprouts. Drizzle another tablespoon of dill sauce over each sandwich. Cover with the remaining four slices of bread.
Wine Recommendation: The grassy, herbal flavors of sauvignon blanc marry beautifully with dill. With this sandwich, the lighter and crisper the wine, the better the match will be. A Sancerre or Pouilly-Fumé from the Loire Valley in France will do nicely.
Notes: If raw onions have too much of a bite, tame them by rinsing the slices under cold running water or soaking them in a bowl of cold water for a few minutes. Pat them dry before putting them on your sandwich.