Step 1
Bring a large pot of water to a boil and season with salt. Add the pasta and cook until very al dente, about 6-7 minutes. Toss in both peas and drain, reserving 1 cup of the pasta water.
Step 2
When the pasta is cooked, place the flour on a plate and season with salt and pepper. Dredge one side of the fish in the flour, shaking off excess. Melt 2 tablespoons of the butter in a large deep skillet over high heat until the butter is just starting to brown. Add the fish flour-side down and cook 2 minutes. Turn and cook another 30 seconds and remove to a plate.
Step 3
Add the drained pasta and peas to the same skillet and add half the pasta water, half the Parmesan and half the lemon zest. Stir in 1 tablespoon lemon juice and toss until combined, adding some of the reserved pasta cooking water to make a sauce. Season with salt, pepper and cayenne, break the fish up into bite sized pieces and add along with the remaining Parmesan, more zest and juice if needed and the remaining tablespoon of butter. Serve with additional Parmesan on the side.
