Cook the pasta. Bring a large pot of salted water to a boil and cook the pasta until al dente. Before draining, save about 1 cup of pasta water. Drain the pasta and set it aside.
Season the salmon. Pat the salmon dry and season it lightly with salt and pepper.
Cut it into bite-size pieces if you have not done that already.
Sear the salmon. Heat the olive oil in a large skillet over medium heat. Add the salmon pieces and cook for about 2 to 3 minutes per side, just until lightly golden and cooked through. Remove the salmon from the skillet and set it aside.
Build the sauce. In the same skillet, add the butter, shallot, and garlic.
Cook for about 1 minute until fragrant. Pour in the broth, then the cream, lemon juice, and lemon zest. Stir well and let it simmer for 2 to 3 minutes.
Add the flavor. Stir in the dill, Parmesan, salt, pepper, and red pepper flakes if you want a little heat.
Taste the sauce and adjust the seasoning if needed.
Wilt the spinach. Add the spinach and stir gently until it softens, which usually takes less than a minute. If the sauce feels too thick, add a splash of the reserved pasta water.
Toss everything together. Add the cooked pasta to the skillet and toss until the noodles are coated. Return the salmon to the pan and fold it in carefully so the pieces stay intact.
Serve right away. Spoon the pasta into bowls and finish with extra dill, Parmesan, or a little more lemon zest if you like a brighter flavor.