Step 1

Preheat the oven to 400 F.

Step 2

For the vegetables. Part 1: Place the sliced zucchini in a large bowl and generously toss with salt. Set aside to allow the salt to draw out liquid from the zucchini.

Step 3

Heat a large skillet to medium-high heat, melt the butter and add the onion and bell pepper along with a pinch of salt and pepper. Sautéed until the onions begin to lightly brown and soften, about 6-8 minutes. Remove from heat and set aside.

Step 4

For the white sauce. In a medium saucepan or skillet, melt the butter over medium heat and then, using a whisk, stir in the flour until it is incorporated into the butter. Reduce the heat to medium-low.

Step 5

Then slowly add in the milk, whisking vigorously to prevent lumps from forming. Continuously whisk until the sauce thickens into a creamy, smooth sauce *(see first note). Finally, whisk in the nutmeg and a pinch of salt and pepper. Remove from heat.

Step 6

For the vegetables. Part 2: Place the zucchini slices on a clean kitchen towel (or thick paper towels) and pat to dry.

Step 7

Assemble the gratin. In an 9x13 casserole dish, start by layering the ingredients in this order: one-third of the zucchini slices, a quarter cup of panko, half of the onion and pepper mixture, and half of the white sauce. Repeat this order again and then top with the remaining one-third of the zucchini slices.

Step 8

For the topping.Combine together the remaining half cup of panko with the Gruyère and sprinkle evenly over the zucchini casserole. Finish by doting the top with small pieces of butter.

Step 9

Bake time. Bake for 25-30 minutes or until the cheese is melted and golden-brown. Let it cool slightly before serving.

Step 10

Best served immediately.