Chicken Spaghetti Squash Casserole* Recipe

Roast spaghetti squash

Preheat oven to 400°F (200°C). Halve the spaghetti squash lengthwise and scoop out seeds. Brush cut sides with 1 tablespoon olive oil and sprinkle with 1/2 tsp salt and 1/2 tsp black pepper. Place cut-side down on a rimmed baking sheet lined with parchment. Roast 35–45 minutes until tender and strands pull away easily with a fork. Remove and let cool until comfortable to handle, then use a fork to scrape out strands into a large bowl. Reserve any liquid in the cavity and discard skin.

If using refrigerated or leftover cooked chicken, shred or dice to measure 3 cups. If starting from raw chicken, roast or poach 2–3 boneless skinless chicken breasts (about 1.5–2 lbs) seasoned with a pinch of salt and pepper until internal temp reaches 165°F (74°C), then shred. Toss shredded chicken with remaining 1 tablespoon olive oil and 1/2 tsp salt and 1/2 tsp black pepper; set aside.

In a large skillet over medium-high heat, add 1 tablespoon olive oil and 1 tablespoon butter. When hot, add diced onion and sauté 3–4 minutes until translucent. Add sliced mushrooms and cook 5–6 minutes until they release liquid and begin to brown. Add diced bell peppers and cook 4–5 minutes more until softened but still slightly crisp. Stir in minced garlic and cook 30–45 seconds until fragrant. Season vegetables with 1 tsp salt and 1 tsp black pepper. Remove from heat and transfer vegetables into the bowl with the scraped spaghetti squash and shredded chicken; toss gently to combine.

In a medium saucepan over medium heat, melt 3 tablespoons unsalted butter. Whisk in 3 tablespoons gluten-free flour and cook, stirring constantly, 1–2 minutes to remove raw flour taste but not brown. Gradually whisk in 2 cups whole milk and 1 cup chicken broth in a steady stream to avoid lumps. Bring mixture to a gentle simmer, whisking frequently, until sauce thickens, about 4–6 minutes.

Once thickened, whisk in 1 teaspoon Dijon mustard, 1 cup grated Parmesan, 1 teaspoon salt, 1 teaspoon black pepper, and 1/4–1/2 teaspoon ground nutmeg (optional). Taste and adjust seasoning. If sauce is too thick, add up to 1/4 cup additional milk or broth to reach a creamy pouring consistency.

Preheat oven to 375°F (190°C) if not already warm. In the large bowl with spaghetti squash, chicken, and vegetables, pour the prepared white sauce and gently fold until everything is evenly coated. Stir in 1 cup shredded mozzarella and 1/2 cup of grated Parmesan (reserve the rest for topping).

Transfer mixture to a lightly greased 9x13-inch (or similar) baking dish and smooth the top. Sprinkle remaining 1/2–3/4 cup shredded mozzarella and 1/4 cup grated Parmesan evenly over the top. If using, mix the 1/2 cup gluten-free breadcrumbs or panko with a teaspoon of olive oil and sprinkle over cheese for a crisp topping.

Bake uncovered in preheated 375°F (190°C) oven for 20–25 minutes until bubbly and golden on top. If you want a more browned top, place under broiler 1–2 minutes—watch closely to avoid burning. Remove from oven and let rest 5–10 minutes to set.

Garnish with chopped fresh parsley if desired. Serve hot.