Heat the oven to 350 F and butter a 1 1/2-quart baking dish.

Melt 4 tablespoons of butter in a large skillet over medium heat. Add the chopped onion and cook, stirring, for 2 minutes. Add the sliced zucchini, cover the pan, and reduce the heat to low. Cook for 8 minutes or until the zucchini is just tender.Remove the zucchini and onions to the baking dish with a slotted spoon.

Wipe out the skillet and add 2 tablespoons of butter; melt it over medium heat and stir in the flour. Cook for 2 minutes then gradually add the milk. Taste and add the salt, mustard powder, black pepper, and 1/2 cup of the cheese. Cook, stirring, until hot and thickened. Pour the mixture over the zucchini mixture in the baking dish.

Melt the remaining 2 tablespoons of butter and toss with the panko crumbs.

Sprinkle the remaining cheese over the casserole, followed by the buttered panko crumbs.

Bake for 25 minutes or until the topping is browned and the casserole is bubbling around the edges.