Step 1: Preheat the Oven

• Set the temperature: Preheat your oven to 400°F (200°C).

Step 2: Sauté the Aromatics

• Melt butter: In a large skillet over medium heat, melt the butter.
• Cook the onions: Add the chopped onion and sauté for 5 minutes until soft.
• Add the garlic: Stir in minced garlic and cook for 1 more minute.

Step 3: Cook the Zucchini

• Add zucchini slices: Stir in the sliced zucchini, salt, pepper, and nutmeg.
• Sauté until tender: Cook for about 8–10 minutes, stirring occasionally.

Step 4: Make the Cheese Sauce

• Pour in the cream: Add the heavy cream and bring to a simmer.
• Stir in the cheese: Remove from heat and mix in the Gruyère and Parmesan cheese until melted.

Step 5: Assemble the Dish

• Transfer to a baking dish: Pour the creamy zucchini mixture into a buttered baking dish.

Step 6: Make the Topping

• Mix the breadcrumbs: In a small bowl, combine panko breadcrumbs with olive oil.
• Sprinkle on top: Evenly spread the breadcrumb mixture over the zucchini.

Step 7: Bake the Zucchini Au Gratin

• Bake until golden: Place in the oven and bake for 20–25 minutes, until the top is golden brown and bubbly.

Step 8: Serve

• Cool slightly and enjoy: Let rest for a few minutes before serving warm.