Place the butter and the oil in a saucepan on very low heat until melted. Don’t let it boil.
Add the flour and cook it, mixing it with a wire whisk until fragrant but not brown (no more than fifteen minutes).
While the flour mixture is cooking, warm up the milk in one saucepan, and the steaming liquid with the vegetable cube in the steaming pot, so that the cube melts. Keep both milk and liquid on very low heat until ready to use.
When the flour mixture has become fragrant, pour on it one cup of the steaming liquid while whisking, then the cup of hot milk. If too thick, add more steaming liquid. Keep whisking until smooth.
Stirring with a wooden spoon, add both cheeses, the garlic and salt and pepper to taste.
Cover the cauliflower/potato mixture with the sauce, sprinkle extra Jarlsberg/Parmesan on top and bake at 350 degrees F until bubbly and hot (about 1/2 hour). Turn oven to broiler just until lightly browned on top.