In a pie pan (or bowl) combine cornmeal, freshly grated Parmesan, salt, and pepper. Set aside.
Crack egg, placing the white into a second pie pan.
Spread mustard over fish. Dip fish first into the egg white and then into the cornmeal mixture. Place onto a plate.
Place oil in a large skillet and heat over medium heat.
Add fish to hot skillet, cooking 2-5 minutes per side until cooked through (exact time will vary depending on thickness of fish).
Warm buns in the oven if desired. Slice tomato.
Spread mayonnaise over the bottom half of each bun; add fish, spritz with fresh lemon juice, and top with lettuce and tomato slices.
Serve sandwiches with celery sticks and grapes on the side.