Preheat the oven to 350 degrees.
Add the sliced sausage to a skillet over medium high heat, and saute until seared. Remove and set aside.
Melt the butter in a pot over medium heat.
Stir in the onions, and saute until softened.
Mix in the garlic, and saute for 30 seconds.
Add in the cream cheese until melted.
Pour in the heavy whipping cream, and whisk to combine well.
Simmer over medium low heat until thickened.
Stir in the shredded cheese, and season with salt and pepper to taste.
Combine the cauliflower, broccoli and sausage in a mixing bowl.
Pour the cheese sauce into the mixing bowl, and stir to combine.
Spread the mixture into a 9x9 baking dish.
Bake for 15-20 minutes.