Bring a large pot of salted water to a boil and add the penne pasta, cooking according to package directions until al dente. While the water heats and pasta cooks, finely dice the onion, mince the garlic cloves, and chop the fresh parsley. Having everything prepped and ready will let you work quickly once the sausage hits the pan.

Heat the olive oil in a large skillet over medium-high heat. Break the Italian sausage into bite-sized pieces and add it to the skillet along with the diced onion, stirring frequently until the sausage is fully cooked and browned, about 8-10 minutes. Add the minced garlic and red pepper flakes, cooking for another minute until fragrant. I like to let the sausage get a nice golden crust before draining—it develops deeper flavor that carries through the whole dish.

Carefully drain off any excess fat from the cooked sausage mixture using a spoon or by tilting the skillet. Pour in the heavy cream and reduce the heat to low, stirring gently to combine. Let this simmer for 2-3 minutes, allowing the cream to warm through and begin to meld with the sausage flavors.

Add the fresh baby spinach to the cream sauce, stirring constantly for about 2-3 minutes until it's completely wilted and darkened. Once the spinach is fully incorporated, remove the skillet from heat and stir in the freshly grated parmesan cheese until smooth and creamy. I find that taking the pan off the heat before adding the cheese prevents it from getting grainy and gives you a silkier sauce.

Drain the cooked pasta from Step 1 and add it directly to the skillet with the cream and sausage sauce, tossing gently until every piece of pasta is coated. Stir in the lemon zest to brighten the flavors, then divide among serving bowls and top with the fresh chopped parsley.