01 - Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions. Reserve 1/2 cup pasta water, then drain with a colander.
02 - While pasta cooks, heat olive oil in a large skillet over medium heat. Add onion and sauté until soft and translucent, about 4 minutes. Stir in garlic and cook 1 minute longer.
03 - Add pumpkin purée, vegetable broth, heavy cream, nutmeg, black pepper, and salt to the skillet. Simmer gently for 5 to 7 minutes, stirring occasionally, until slightly thickened. Incorporate Parmesan cheese, stirring until sauce is smooth. If sauce thickens too much, adjust with reserved pasta water.
04 - In a small saucepan over medium heat, melt unsalted butter. Add sage leaves and cook, swirling occasionally, until butter is golden brown and sage is crisp, 2 to 3 minutes. Remove from heat.
05 - Add cooked pasta to the pumpkin sauce. Toss gently to ensure even coating. Warm through, using reserved pasta water as needed for consistency.
06 - Divide pasta among plates. Drizzle with sage brown butter and top with crispy sage leaves. Garnish with additional Parmesan, cracked pepper, and toasted pumpkin seeds if desired.