Preheat your oven to 425°F (220°C).

In a large skillet over medium heat, melt two tablespoons of butter along with one tablespoon of olive oil. Sauté the finely chopped onion until soft and translucent, about 5–7 minutes.

Add the sliced zucchini, salt, and black pepper to the skillet. Cook for about 10 minutes until the zucchini is tender but maintains some structure.

Return the drained zucchini to the skillet. Sprinkle with one tablespoon of flour and stir to coat. Gradually add ½ cup of milk, stirring until slightly thickened, about 3–4 minutes.

Transfer the mixture into your baking dish, spreading it evenly. Top with 2-3 tablespoons of Gruyère or Swiss cheese and 1-2 tablespoons of breadcrumbs.

Bake in the preheated oven for 20-25 minutes until the top is lightly browned and crisp.

Let the gratin rest for a few minutes before serving.