How to Make It
In a small bowl, stir together the yogurt, dill, mustard, oil, vinegar, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
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.
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.