Cook the pasta: Begin by cooking the pasta in a large pot of well-salted boiling water. Cook it until it is just one minute shy of al dente. For the best results, add the pasta to the water once the mushrooms have sautéed for about three minutes. Before draining, reserve one cup of the starchy pasta water for later use.

Sauté the mushrooms: While the pasta is cooking, melt the butter in a large non-stick pan over medium-high heat. Add the sliced mushrooms and cook them for 6 to 8 minutes, stirring occasionally, until they become golden brown. Next, stir in the garlic, rosemary, and crushed red pepper flakes. Continue sautéing for an additional 2 minutes to allow the flavors to meld together.

Combine the ingredients: Pour in the heavy cream, Parmesan, lemon juice and zest, along with 1/4 cup of the reserved pasta water. Stir until all the ingredients are thoroughly mixed. Add the spinach and tuna, then incorporate the cooked pasta, tossing everything gently until the spinach wilts.

Season to taste: If the pasta appears too dry, add a bit more of the reserved pasta water or a splash of extra cream to achieve the desired consistency. Taste the dish and adjust seasoning with salt and freshly cracked black pepper as necessary.