Preheat oven to 400 degrees.
Bring a pot of water to a boil. Salt water and add brussels sprouts. Cook for 5-7 minutes until just tender and bright green.
While brussels sprouts cook, heat olive oil over medium heat in an oven proof skillet. Add onion and sauté for 5 minutes. Add garlic and seasoning and cook for 30 - 40 seconds longer.
Sprinkle flour over onions and stir to coat. Stir in broth and mix until thickened a bit.
Pour in milk and stir until smooth. Add salt and pepper and stir.
Once the brussels sprouts are tender, drain and add to the skillet. Stir to coat in sauce and spread in an even layer.
Sprinkle desired amount parmesan over brussels sprouts.
Transfer skillet to oven and bake until sprouts are tender, about 15 minutes.
Cool briefly and serve.