Preheat your oven to 400°F. While the oven heats, fill a large pot with salted water and bring it to a boil—this should be ready to go when you finish your prep work. Thinly slice the shallots into rings and cut the salmon into 1-inch cubes for even cooking. Measure out the spinach and have the Boursin cheese ready to use. This prep work ensures smooth cooking once you start the main components.
In a large baking dish or 9x13 inch casserole, combine the olive oil, spinach, and sliced shallots, tossing gently to coat the spinach with oil. Create a small mound in the center and place the entire block of Boursin cheese there. Arrange the salmon cubes around the cheese in a single layer. Drizzle the salmon with a bit more olive oil and season generously with salt, pepper, and garlic powder. I like to season the salmon separately from the base because it ensures even flavor distribution on the fish itself.
Cover the baking dish tightly with foil or a lid and place it in the preheated 400°F oven. Roast for 20-25 minutes until the salmon is cooked through and flakes easily with a fork, and the spinach has wilted. While this roasts, cook your fettuccine in the boiling salted water according to package directions until al dente. Reserve 1 cup of pasta water before draining—this starchy water is crucial for creating a silky sauce. I always reserve the pasta water before draining because once it's gone, you can't get it back!
Remove the baking dish from the oven and let it cool for 2 minutes. Using a fork or wooden spoon, break apart the salmon cubes and gently mash them together with the Boursin cheese, spinach, and shallots, creating a creamy mixture. The residual heat will soften the cheese and help it emulsify with the salmon juices. Add the cooked fettuccine from Step 3 to the baking dish and toss gently. Start with 1/2 cup of the reserved pasta water from Step 3 and mix until you reach a silky, cohesive consistency—add more pasta water if needed. Season the final dish with salt, pepper, and fresh lemon juice to brighten the flavors.
Divide the salmon pasta between serving bowls or plates. Garnish generously with fresh parsley and serve immediately while still warm. The vibrant green parsley adds a fresh contrast to the rich, creamy sauce.