1) Heat oven to 450 degrees if your fish fillets are more than a 1/4 inch thick. Per package instructions, bring water to boil, add pasta, and cook until just barely al dente. Drain, reserving at least 2 cups of the cooking water.
2) Meanwhile, add one teaspoon light olive oil to a large skillet. Cook thinly sliced onions over medium-high heat until caramelized and browned, and transfer to a plate. Then add one tablespoon of oil to the skillet and cook the fish fillets over high heat for about 2 minutes or until opaque. If your fish fillets are more than a quarter-inch thick, transfer them to an oven-safe dish, drizzle with a little more olive oil, and bake them in the oven until a sharp knife can easily penetrate the deepest part of the fish. Be careful not to overcook. Season with salt and pepper. Transfer cooked fish to the plate.
3) Add the remaining oil to the skillet and place over medium heat. Add the crushed red pepper flakes and the lemon zest. Whisk in 1 cup of the reserved pasta water. Toss in the cooked pasta, spiraled zucchini, lemon juice, grated cheese, pepper and salt to taste. Cook until pasta and zucchini are well-coated with sauce. Add more pasta water if needed. Add the caramelized onions and taste. If the pasta seems too sour, balance the lemon by adding 1/2 - 1 tsp real maple syrup or honey over it and toss through until well-combined. I've only had to do this when my onions are not as sweet as usual. Add fresh parsley just prior to serving. Top with more grated cheese and serve with the fish. Enjoy!