In a medium skillet over medium-medium high heat, saute the onion in a little oil. Cook till the onion is translucent. Season with salt.
Oil or butter a baking dish, and layer half of the zucchini slices in. Some slices will be overlapping. Salt and pepper the layer.
Put the sauteed onions on top of the zucchini layer, then add half of the cheese in a layer.
Layer the rest of the zucchini slices over the cheese layer and season with salt and pepper.
Put the rest of the cheese on the zucchini, then pour the milk over the whole dish.
Mix the breadcrumbs with the melted butter and put the mixture on top of the last cheese layer.
Bake for 30 minutes or until it's tender and toasty.
Notes
The recipe calls for Gruyere cheese, but you could substitute Emmental or even sharp cheddar. I feel that the sharpness is needed to balance the subtle flavor of the zucchini.I process homemade sourdough bread that is nearly stale, then dry the crumbs and keep them in the freezer. You can do the same with any bread that you like, even gluten-free versions. You can also use store-bought crumbs or stuffing mix. In my book, nothing beats homemade, though!