Bring a large pot of salted water to a boil.

While water heats, place the butternut squash on a cutting board and carefully pierce it several times with a knife. Microwave on high for 5 minutes to soften the skin slightly, making it easier to peel.

Peel the squash with a vegetable peeler or sharp knife, remove the seeds with a spoon, and cut the flesh into 3/4-inch cubes (you should have about 4 cups).

Add pasta to the boiling water and cook until just al dente, about 2 minutes less than the package direction. Reserve 1 cup of pasta water, then drain.

In the last 2 minutes of pasta cooking, add the squash cubes to the same pot of boiling water so they cook tender.

Melt butter in a large skillet over medium heat. Once foaming, add the sage leaves and cook, swirling occasionally, until the butter turns golden brown and the sage is crispy, about 4 minutes.

Finely mince the garlic and add it to the brown butter, stirring constantly for 30 seconds until fragrant. Be careful not to let it burn.

Drain the pasta and squash together and add to the skillet with the brown butter. Toss gently, adding splashes of reserved pasta water (start with 1/4 cup) until the sauce coats everything lightly and clings to the pasta.

Season with kosher salt and black pepper to taste. Divide among bowls, top with freshly grated parmesan, a pinch of red pepper flakes, and a crispy sage leaf from the pan.