Step 1

Preheat oven to 375 degrees F. Coat two 8-inch-square baking dishes with cooking spray.

Step 2

Halve squash lengthwise and scoop out the seeds. Place cut-side down in a microwave-safe dish; add 2 Tbsp. water. Microwave, uncovered, on High until the flesh can be scraped with a fork but is still tender-crisp, 10 to 12 minutes. Scrape the strands onto a plate; set aside.

Step 3

Place broccoli in the same microwave-safe dish; add 1 Tbsp. water and cover. Microwave on High, stirring occasionally, until just barely tender-crisp, 2 to 3 minutes. Drain and set aside to cool.

Step 4

Meanwhile, heat oil in a large nonstick skillet over medium-high heat. Add mushrooms and cook, stirring, until they've released their juices, about 8 minutes. Add onion and continue cooking until the onion is tender and the mushrooms are lightly browned, about 8 minutes.

Step 5

Stir in garlic, thyme, and pepper; cook, stirring, for 30 seconds, Stir in soup (do not dilute with water) and heat through. Stir in chicken and the reserved squash and broccoli; gently toss to combine well.

Step 6

Divide the mixture between the prepared baking dishes and sprinkle each with 1/4 cup Cheddar. Cover with foil. Label and freeze one casserole for up to 1 month.

Step 7

Bake the remaining casserole until bubbling, about 25 minutes. Uncover and continue baking until lightly browned along the edges, 10 to 15 minutes more. Let stand for 10 minutes before serving.