Get the penne started in a large pot of well-salted boiling water, following the package timing for al dente. You'll want it ready right when the sauce comes together.
Meanwhile, drizzle the olive oil into a large skillet and bring it up to medium-high heat. The pan should be hot enough that the oil shimmers but doesn't smoke.
Add the sausage along with the minced onion, garlic, red pepper flakes, and salt. Break up the sausage with your spoon as it cooks, creating bite-sized chunks. Keep everything moving until the sausage loses its pink color completely.
If there's more than a tablespoon of fat pooled in the pan, pour most of it off. A little sausage fat adds flavor, but too much will make your sauce greasy.
Reduce the heat to low and pour in the heavy cream. Let it come to a gentle simmer — you'll see small bubbles around the edges — then add all the spinach at once.
Stir constantly as the spinach wilts down into the cream. It'll seem like too much at first, but it shrinks dramatically as it cooks.
⏱ 3 min
Remove the pan from heat and stir in the parmesan, letting it melt into the sauce from the residual heat. Add the drained pasta and toss until every piece is coated.