Preheat oven to 375 degrees F.

Bring 12 inch skillet to medium-high heat. Add 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 flour and use a whisk to combine with butter to create a paste (aka a 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 and remove from heat. 

Spray a 9x13 casserole pan with olive oil. Layer half of the potatoes on the bottom. Top with half the sauce then 1/2 cup of the monterey cheese (or Gruyere), 1/2 cup of cheddar jack and all of the parmesan. Then layer the remaining potatoes, the remaining sauce and the rest of the monterey jack and cheddar jack cheeses. 

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.