Bring a large pot of salted water to a boil. Add the penne and cook as the label directs. Reserve 1/2 cup of the cooking water, then drain the pasta.
Meanwhile, heat the oil in a large skillet over medium-high heat. Add the sausage and cook, breaking it up into small pieces with a wooden spoon, until browned and cooked through, 3 to 4 minutes. Add the collard greens, garlic, pepper flakes and 1/2 teaspoon salt and cook, stirring occasionally, until the greens are wilted, about 3 minutes.
Add the pasta, lemon zest, and half of the reserved pasta water to the skillet and toss. If needed, add more of the pasta water and toss again. Serve topped with Parmesan.