Melt the butter over medium heat and add the zest and juice of one lemon. Reduce the heat to low.
Meanwhile, start cooking your pasta, as this is a quick sauce. Once your pasta is HALFWAY cooked, add the fiddleheads in to boil with the pasta and cook.
While the pasta and fiddleheads finish cooking, you're going to finish your sauce. Add the milk and parmesan cheese, whisk so it becomes a sauce quickly. Remove from heat.
Drain the pasta and fiddleheads, saving 1/2 cup pasta water.
Add pasta and fiddleheads to sauce, toss and coat the pasta. Add pasta water to loosen sauce if needed only.
Season with salt and pepper to taste, serve with an extra teaspoon of butter to bring it all together.