Bring a large pot of water to a rolling boil. Add halved Brussel sprouts to water and cook for 5-7 minutes until just tender. Transfer Brussels sprouts to a large bowl of ice water to stop cooking. In a medium saute pan melt 2 tablespoons of butter over medium heat and sautee leeks and shallots for about 5 minutes or until tender. In a large bowl combine heavy cream, cheeses, rosemary and salt & pepper. Drain Brussel sprouts and pat dry with a paper towel. Add Brussel sprouts and leek/shallot mixture to cheese mixture. 

In a medium bowl add breadcrumbs, melted butter, and salt & pepper. Grease a casserole dish or 10-inch cast iron skillet. Add brussel sprout mixture to pan/casserole dish evenly and sprinkle breadcrumbs mixture on top. Bake for 30-35 minutes or until mixture is bubbly and the breadcrumbs are golden brown.