In a small bowl, gently mix the crumbs, tarragon, lemon zest, melted butter, and a little salt and pepper.
Spread each fillet with 1/4 teaspoon mustard and season with more salt and pepper. Carefully pat the crumb topping over the surface of each fillet, pressing lightly so it sticks.
Brush a little oil onto a small baking sheet or shallow baking pan and set the fillets on the oiled spot (or use a nonstick pan). Bake the fish in the hot oven until the topping is golden brown and crisp and the fish is tender all the way through when you poke it with a thin knife or a skewer, 10 to 15 minutes.
If the topping seems like it's going to burn before the fish is done, turn the heat down to 375?. Serve immediately.