Roast squash at 425°F for 20 minutes, until fork-tender. Cool slightly and mash or puree.

Cook macaroni until just shy of done; drain and set aside.

Melt butter in a skillet, stir in flour for 1 minute. Whisk in milk until smooth and bubbling.

Stir in cheddar, mozzarella, salt, pepper, and nutmeg until melted.

Mix in mashed squash and cooked pasta. Transfer to slow cooker.

Sprinkle panko on top if using. Cook on Low for 1.5–2 hours, checking at 1 hour to avoid drying.