Preheat the oven to 425°F (220°C). Toss the cubed butternut squash with 1 tablespoon olive oil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper on a rimmed baking sheet in a single layer. Roast for 20–25 minutes, turning once, until edges are caramelized and tender.

Place the halved Brussels sprouts on a second rimmed sheet, drizzle with 1 tablespoon olive oil, season with 1/2 teaspoon salt and 1/4 teaspoon black pepper, and roast for 18–22 minutes until cut sides are browned and leaves are crisp. If you only have one baking sheet, roast Brussels sprouts for the last 18 minutes alongside squash on the same sheet, giving them a corner.

Season the cubed chicken with 1/2 teaspoon salt, 1/4 teaspoon black pepper, 1 teaspoon dried thyme, and 1 teaspoon dried rosemary. Heat a large wide skillet over medium-high heat and add 1 tablespoon olive oil. When the oil shimmers, add the chicken in a single layer (work in batches if needed) and sear without moving for 2–3 minutes so a golden crust forms. Flip and cook another 2–3 minutes until chicken is cooked through and browned. Remove chicken to a plate and tent with foil.

In the same skillet over medium heat, add 2 tablespoons unsalted butter and the sliced onion. Sauté for 5–7 minutes until the onion softens and develops golden edges. Add the minced garlic and 1/2 teaspoon red pepper flakes (if using) and cook for 30–45 seconds until fragrant.

Pour in 1/2 cup chicken broth and scrape the pan with a wooden spoon to lift any browned bits. Bring the liquid to a gentle simmer and stir in 1/4 cup grated Parmesan and 2 tablespoons lemon juice. If you want a slightly thicker glaze, whisk 2 teaspoons cornstarch with 2 tablespoons water to make a slurry and stir it into the simmering broth. Simmer 1–2 minutes until slightly reduced. Taste and adjust seasoning with salt and freshly cracked black pepper.

Meanwhile, cook the tortellini according to package directions until just al dente; if using frozen tortellini, add 1 minute less than package time. Reserve 1/2 cup of the pasta cooking water before draining.

Return the seared chicken to the skillet along with the roasted butternut squash and roasted Brussels sprouts. Gently fold in the cooked tortellini and add 2 tablespoons chopped fresh parsley. If the mixture seems dry, add up to 1/4 cup reserved pasta water to loosen the sauce. Warm everything together for 1–2 minutes so flavors marry and tortellini absorbs some sauce.

Taste and finish with an extra squeeze of lemon, a drizzle of balsamic vinegar if using, and a generous sprinkle of freshly grated Parmesan. Garnish with additional chopped parsley and a few twists of cracked black pepper. Serve immediately in a shallow bowl so the roasted edges remain visible.