Butter one large soufflé dish well (or 4-6 small ramekins) and preheat oven to 350 degrees.
In one saucepan, warm the milk with the onions and cook on low heat for 10 minutes ensuring the mixture never boils. Remove from heat. Strain the milk and discard the onions.
In another saucepan, melt the butter and add the flour. Once combined, remove from heat and gently stir in the onion-flavored milk.
Add the mustard, nutmeg and Castello Havarti Cheeses and continue to stir until the cheese melts. Pour into your soufflé dish. Allow to cool 5-10 minutes.
Add the egg yolks, mixing very well. Beat the egg whites until they form mountain tops. Add the beaten egg whites to the mixture, one spoonful at a time, folding gently into the mixture.
Place the soufflé mix into the preheated oven for 20 minutes, then turn the heat down to 300 degrees and bake for another 10-15 minutes depending on how well-done you prefer your soufflé. If you decide to bake the soufflé in small ramekins, bake for 15 minutes at 350 degrees, and then for 10 minutes at 325 degrees.
Serve immediately with mustard and a side salad (optional).