Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions until al dente. Reserve 1/4 cup of pasta water before draining.

Sauté the garlic and spinach: While the pasta is cooking, heat olive oil in a large skillet over medium heat. Add garlic and sauté for about 30 seconds, until fragrant. Add spinach and cook until wilted, about 2-3 minutes. Season with salt and pepper. Remove spinach mixture from the skillet and set aside.

Cook the salmon: In the same skillet, add the salmon cubes. Season with salt, pepper, and red pepper flakes (if using). Cook for about 3-4 minutes, until the salmon is cooked through and flakes easily with a fork, turning once halfway through. Be careful not to overcook.

Make the sauce: Reduce the heat to low. Add heavy cream, Parmesan cheese, lemon juice, and butter to the skillet with the cooked salmon. Stir until the cheese is melted and the sauce is smooth. If the sauce is too thick, add a little of the reserved pasta water until it reaches your desired consistency.

Combine everything: Add the cooked pasta and spinach mixture to the skillet with the salmon sauce. Toss to combine, ensuring the pasta is well coated.

Serve: Serve immediately, garnished with extra Parmesan cheese and a squeeze of lemon juice, if desired.