Oven: Preheat oven to 425 degrees F. 

Prep Squash: Prepare delicata squash by remove the ends, slicing in half lengthwise and then cutting into 1/2 inch thick half moons. Place in a bowl and toss with olive oil. Season with kosher salt and ground black pepper. Place in rows in an 8x12 baking dish. Place in oven, middle rack, for 15-20 minutes.

Make Sauce: While the squash roasts, bring a cast iron skillet to medium-high heat. Add butter and let melt; swirl to coat the pan. Add onion and garlic. Move around until garlic becomes fragrant and onion starts to soften, approx. 1-2 minutes. Add flour and whisk to combine with butter to create a paste (aka a roux). Continue whisking while adding the creamer and then broth, approx. 1/4 cup at a time. Continue whisking and letting it simmer until consistency is thick and creamy. If it feels too thick, add another splash or two of broth and stir. 

Add Cheese: Remove from heat and add parmesan and 1/2 cup of the Gruyere cheese. Stir until cheese has melted. Taste and add any salt or pepper, as desired. 

Bake: Pour creamy sauce over the roasted delicata squash (after it's done baking) and then sprinkle with remaining 1/2 cup of Gruyere. Return casserole dish to the oven, middle rack and uncovered, for 8-10 minutes. Then move turn on the oven broiler for 1-3 minutes (I would recommend leaving the door open so you can watch carefully) or until sauce is bubbly and starting to turn golden brown. 

Cool + Serve: Remove and let cool 10 minutes before serving. Enjoy!