Prepare your ingredients first! Clean the outer dark and/or dry leaves off the brussels sprouts and cut off the bottom of the stem. Cut each brussels sprout in half or in quarters. Slice the onion thinly and smash and mince the garlic. Measure remaining ingredients.
Preheat the oven to 400° and grease a 2 quart baking dish (or similar size).
Preheat a cooking pan over medium heat and add some olive oil.
Start cooking the onions for a minute or so and then add brussels sprouts. Let brussels sear until nicely browned. Toss/mix them once in a while but not too often to let them get a nice sear.
Once seared, stir in the garlic, mix and let it cook for just a few seconds.
Take everything out of the pan and into the prepared baking dish.
Lower the heat under the pan for medium-low.
Melt butter and sprinkle the flour over it while whisking constantly. Once incorporated, start SLOWLY pouring in the broth and then cream, while constantly but slowly whisking.
Whisk in Dijon mustard, parmesan cheese, salt and pepper. Let everything melt and incorporate smoothly and take off heat.
Pour the cheese sauce over the brussels sprouts in the baking dish and stir it together to coat the brussels evenly.
Sprinkle a little more parmesan cheese over the top and bake for about 10-13 minutes.