Preheat oven to 400 degrees F. 

Prepare brussels sprouts by trimming off the stem and peeling off 2-3 outer layers of leaves. Quarter each brussels sprout and place in a 8x12 casserole dish. 

Season with kosher salt and ground black pepper. Drizzle with olive oil. Massage oil into the brussels sprouts. Place in oven, middle rack, for 20 minutes, tossing halfway.

Meanwhile, 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 until consistency is thick and creamy. If it feels too thick, add another splash or two of broth and stir. 

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

Pour sauce over roasted brussel sprouts 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 to the 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 brown. 

Remove and let cool 10 minutes before serving.