Preheat the oven to 375ºF.

Prepare the cheese sauce: Place the milk in a small saucepan and set over medium low heat.

Meanwhile, place the olive oil in a separate medium saucepan and set over low heat. Add the flour to the pot of olive oil, stirring constantly until the mixture begins to bubble, about 2 minutes. Turn off the heat. Very slowly, whisk in the simmering milk into the flour mixture until the milk has been fully incorporated to the mixture. Change to a wooden spoon and stir in the salt, pepper, nutmeg, and grated Swiss cheese. Continue stirring until the cheese is completely melted. Taste for seasoning (if it tastes bland, your gratin will taste bland so add more spices as needed). Set aside.

Bake the broccoli mixture: Pour half of the sauce on the bottom of a baking dish. Place the broccoli on top and then pour over the rest of the sauce. Gently fold the broccoli into the sauce so that the vegetable is evenly coated.

Sprinkle the Parmesan and panko breadcrumbs evenly over the top of the broccoli. Bake for 15-20 minutes. Turn on the broiler and broil for 5 minutes or until the top is golden brown.

Serve: Serve warm.

Cooking Tip: In order to achieve a sauce with a velvety thick texture, it is very important that you add the milk very slowly to the flour mixture.