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°F 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.
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Notes
*Gluten free note: to make this recipe gluten free, simply substitute regular flour with a 1:1 gluten free all purpose flour. (It's best not to use almond flour or coconut flour, results will not be the same.)
Heavy whipping cream: heavy whipping cream provides the best rich and creamy texture to sauces, so I always recommend using it. If you are in a pinch, you can use half and half but note that it won't be quite as thick and creamy. If you wish to make the sauce thicket, you can add another 1/2 tbsp or less of flour.
Parmesan cheese: Use freshly grated parmesan cheese for the best flavor and consistency. Freshly grated parmesan cheese will also melt much smoother.