Preheat oven to 375 degrees F.

Bring a medium-sized skillet to medium-high heat. Add 2 tablespoons butter and let melt. Swirl to coat the pan. Add garlic and onion. Move around pan until garlic becomes fragrant and onions soften a bit, approx. 2-3 minutes. Remove onions and place on a small plate; set aside. 

Add the other 2 tablespoons butter and let melt. Add flour and use a whisk to combine with melted butter to create a paste (aka a homemade roux). Whisk continually and slowly add the creamer about 1/4 cup at a time. Continue adding and whisking as the liquid combines with the roux. Once you have added the creamer, add the broth the same way. Continue whisking and ensure the mixture is simmering gently as it continues to thicken. Add salt, pepper, and onions back in, stir to combine.

Remove from heat and add in 1/2 the Gruyere, 1/2 the monterey jack and all of the parmesan cheese. Stir to combine as it melts. 

Use a large 12inch cast iron skillet (approx. 2 inches deep) and layer half of the thin slices of potato on the bottom. Top with half the cheese sauce then layer the remaining potatoes, the remaining sauce and the rest of the Gruyere and monterey jack cheeses (use more cheese, as desired on top). 

Cover with aluminum foil and place in oven, middle rack, for 1 hour. 

Remove and test the potatoes ensuring they are soft, but still a bit firm. If they need longer, cover again continue cooking in 10 minute increments until they are fully cooked. Otherwise, uncover, and return to oven for 10 minutes to really make sure the cheese is bubbly. 

Remove from oven and let cool at least 15 minutes (otherwise it will fall apart if you try and scoop it up) before serving.