Preheat oven to 425F and grease a casserole pan. Dice the yellow onion and mince the garlic cloves. Trim the ends off of brussels sprouts and cut into quarters or eighths depending on the size.

Heat oil in a large skillet over medium-low heat and saute the diced onion and minced garlic until soft and fragrant, about 5 minutes. Add the cut brussels sprouts to the pan and increase the heat to medium. Season the brussels sprouts with 3/4 tsp of salt and saute them for 8 minutes, tossing frequently.

Pour this mixture into a casserole dish and return the pan to the heat. Melt butter in the pan and then mix flour into the melted butter until a paste remains. Slowly pour the heavy cream in the pot mixing constantly until a roux forms.

Mix in the vegetable broth, 1 cup gruyere cheese, 3/4 tsp salt and 1/2 tsp pepper and continue mixing until all the cheese is melted. Pour cheese sauce over the brussels sprouts in the casserole dish.

Top with another cup of gruyere cheese and place in the oven to bake. Bake at 425F for 23-25 minutes or until the top is golden brown and bubbling