Add 1 T of the butter into a skillet on medium-high heat.
Once the butter is melted, add the chopped onion. Saute the onion for 2-3 minutes until it softens and browns.
Add the halved Brussels sprouts and mix in 1 teaspoon of garlic pepper. Add salt and pepper.
Pour the mixture into a baking dish.
Using the same skillet, add 1 cup of the heavy whipping cream. Bring it to a boil, and then add the shredded Havarti cheese. Mix until the cheese is melted and smooth.
Add 3/4 cup of the grated Parmesan cheese and whisk until smooth and thick.
Pour the cheese mixture over the top of the Brussels sprouts in the casserole dish and stir.
Add 1/4 cup of the shredded parmesan chees and put the casserole dish in the oven.
Bake for 20-25 minutes until the top is brown and the cheese sauce is bubbling. The Brussels should also be fork tender.