Preheat oven to 375 degrees.
Bring large pot of salted water to a bowl.
Carefully add cauliflower florets & brussels sprouts (that have been trimmed and cut in half) to boiling water.
Cook about 3-4 minutes and then drain and place in ice water bath to stop cooking.
Drain veggies again and put in shallow baking dish.
Meanwhile in sauce pan on stove top melt butter over medium low heat. Stir in cornstarch till combined and then add chopped onion.
Mix until it is slightly thickened about 2-3 minutes.
Add half & half cream and stir. Increase heat to medium high and continue to cook(stirring continually) till starting to thicken about 5-6 minutes.
Mix in 2 cups gruyere cheese & ¼ cup of parmesan cheese and stir till completely melted.
Pour over veggies and combine. Top with remaining 1 cup gruyere cheese and ¼ parmesan cheese and bake in preheated oven approximately 30 minutes till it develops a nice brown crust.
Allow at least 10 minutes prior to serving.