1. Preheat the oven to 425 degrees F.

Prepare the Brussels Sprouts:

1. Prepare an 8×8 inch baking dish by brushing the insides with some of the melted butter. Add the brussels sprouts, remaining melted butter, 1/2 tsp salt and ⅛ tsp pepper. Toss to coat.
2. Spread the brussels sprouts out evenly and roast until almost fork tender, about 15-20 minutes, tossing once halfway through baking time. Remove from oven and set aside.
3. Reduce oven temperature to 375 degrees.

Make the Sauce:

1. In a medium sized sauce pan, melt butter over medium heat, do not let brown.
2. Whisk in the flour to make a roux, and cook, whisking continuously, until blonde in color, about 1 1/2 minutes.
3. Slowly whisk in the half and half and increase heat to medium high and bring mixture to a boil. Reduce heat to a gentle simmer over medium low heat. Add sprigs of thyme. Cook until thickened, about 15 minutes, stirring occasionally.
4. Season with nutmeg, salt and pepper. Remove thyme sprigs with a slotted spoon and discard. Turn heat off and stir in gruyere cheese. Taste and adjust seasoning. Set aside.

Prepare the Onions:

1. Heat olive oil in a sauté pan over medium heat. Add the pearl onions and season with salt and pepper. Sauté until light golden in color, about 7 minutes. Remove from heat and set aside.

Prepare the Toping:

1. Combine all ingredients in a small mixing bowl. Set Aside.

To Assemble:

1. Add the pearl onions to the baking dish containing the brussels sprouts and stir to mix. Pour the sauce evenly over the vegetables.
2. Sprinkle the breadcrumb mixture evenly over top.
3. Bake uncovered, 20 minutes, or until sauce is bubbling and top of gratin is golden brown. Allow to rest 5-10 minutes before serving.