Preheat the oven to 350°F. Spray a baking dish with cooking spray. Arrange the haddock fillets in a single layer in the baking dish, and set aside.
Meanwhile, melt the butter in a small dish (I usually use my glass measuring cup) in the microwave. Add in the breadcrumbs, parsley, lemon zest, and a pinch of Kosher salt. Toss with a fork until well combined.
Brush the haddock fillets with a thin layer of mustard, and then top with the breadcrumb topping. Bake uncovered for 10 – 12 minutes, until the fish is white and flakes easily with a fork. Serve immediately.