Preheat the oven to 425 degrees . Put the egg whites in a deep plate or shallow bowl. On another plate, pour the butter over the panko and work into the crumbs to evenly distribute. Season the crumbs with the thyme, chives, Old Bay, onion powder, garlic powder and grated peel of 1 lemon. Season the fish with salt and pepper and coat in the egg whites, then in the crumbs; arrange on a slotted broiler pan or wire rack on top of a baking sheet. Bake the fish until deep golden and crisp, about 15 minutes.
While the fish bakes, grate the shallot into a shallow bowl and add the juice of the grated lemon and the mustard. Whisk in the EVOO; season with salt and pepper. Add the zucchini, tomatoes and scallions to the dressing and toss to coat.
Cut the second lemon into wedges and serve with the fish. Pile the zucchini salad alongside.