Prep the squash and sprouts: Peel the butternut squash, remove seeds, and cut into 1-inch cubes. Trim the Brussels sprouts and halve them through the stem. Pat both dry with paper towels; moisture will prevent proper browning.

Toss squash with 1 tablespoon olive oil, a generous pinch of salt, and a few grinds of black pepper. Arrange in one layer on a rimmed baking sheet. Roast at 425°F (220°C) for 20–25 minutes, flipping once until cubes are golden and tender.

Toss halved Brussels sprouts with 1 tablespoon olive oil, salt, and pepper. Add them to the same baking sheet for the last 12–15 minutes of the squash’s roasting time so they can develop a crisp, caramelized surface.

Season and sear the chicken: Pat the chicken pieces dry and season with salt, pepper, and half the dried thyme. Heat a large oven-safe skillet over medium-high heat and add 1 tablespoon olive oil. When shimmering, add the chicken in a single layer and sear until well browned on all sides and cooked through, about 5–7 minutes. Transfer the chicken to a plate and tent with foil.

Sauté aromatics: Reduce heat to medium and add the unsalted butter to the skillet. When melted, add the sliced onion and cook until translucent and starting to brown at the edges, about 6 minutes. Add the minced garlic, remaining thyme, and red pepper flakes (if using) and cook another 30–60 seconds until fragrant.

Make a quick pan sauce: Sprinkle the tablespoon of flour over the onions and garlic and stir for 30 seconds (optional). Pour in 1 cup chicken broth while scraping up browned bits from the bottom of the pan. Bring to a gentle simmer and let the liquid reduce slightly for about 2–3 minutes.

Add tortellini: Add refrigerated tortellini directly to the skillet and nestle into the simmering broth so it can heat through; cook until tender according to package directions, usually 3–5 minutes. If using frozen tortellini, add as directed or boil separately and reserve 1/2 cup pasta water to loosen the sauce if needed.

Return chicken and vegetables to the pan: Add the roasted butternut squash, Brussels sprouts, and seared chicken back into the skillet, folding gently so tortellini remain intact. Warm through for 1–2 minutes so flavors meld.

Finish with brightness and cheese: Remove the pan from heat. Stir in the lemon zest and juice, 1/2 cup grated Parmesan, and chopped parsley. Taste and adjust seasoning with salt and cracked black pepper. If the sauce is too thin, simmer a minute longer; if too thick, add up to 1/4 cup reserved pasta water or an extra splash of broth.

Serve immediately: Sprinkle additional Parmesan and chopped parsley over individual portions and offer lemon wedges at the table. Serve family-style in the skillet or plate into shallow bowls.