Preheat oven to 375 degrees

Meanwhile bring large pot of salted water to boil.

Remove outer leaves from brussels sprouts, trim bottom and slice in half.

Put brussels sprouts in boiling water and cook about 5-7 minutes(you just want to blanch them).

Drain and place in casserole dish.

Meanwhile melt butter in sauce pan over medium heat.

Add corn starch and chopped onion and stir a couple minutes to form a roux.

Add half & half cream and stir frequently till slightly thickened 3-5 mins.

Add 2 cups of Grueyere cheese and stir until melted.

Pour over brussels sprouts and stir to combine.

Top with additional Grueyere cheese and place in preheated oven.

Bake 25-30 minutes until cheese is bubbly and golden brown.