Preheat oven to 400 degrees F.
In a small bowl, mix the buttermilk, lemon juice, feta cheese and dill. Blend well. Allow flavors to blend in the refrigerator for at least 15 minutes.
On a clean work surface, sprinkle both sides of the chicken with the oregano, garlic salt and lemon pepper.
Heat a heavy nonstick pan over medium-high heat. Add the olive oil and sear chicken for about 1 1/2 minutes on each side. Reserve.
Mix the panko and Parmesan in a shallow dish. Using a basting brush, paint the chicken with hot mustard. Dredge the chicken through the panko mixture.
Place the chicken on a wire rack that is sitting on top of a baking sheet. Bake the chicken until panko mixture is browned and crispy, about 5 minutes.
Spread buttermilk mixture onto the sliced face of each of the rolls. Top each roll with a chicken breast, 1/4 of the lettuce and 1 slice of tomato. Serve open face with a little more of the buttermilk sauce drizzled on top of the sandwich.