2 pounds zucchini, cut diagonally into 1/4-inch slices
3 tablespoons unsalted butter
1 large onion, halved and thinly sliced
freshly ground pepper
8 ounces chevre, at room temperature
1/2 cup heavy whipping cream
1 ounce Parmesan cheese, freshly grated (1/4 cup)
1/4 cup dry bread crumbs
Preheat oven to 350 degrees. Grease a 9 by 13-inch baking dish. Boil zucchini in salted water until tender, but still crisp, about 2 minutes. Drain and pat dry.
Melt butter in a skillet. Add onion and sauté until translucent. Arrange one-third of zucchini in prepared dish. Season with salt and pepper. Spread half of onion over zucchini. Repeat layers, seasoning with salt and pepper, and end with zucchini on top.
Crumble chèvre into top of a double boiler. Stir in cream. Cook until smooth and thick, 3 to 4 minutes. Thin with an additional 2 to 4 tablespoons cream if too thick. Pour over zucchini layers. Combine Parmesan and bread crumbs. Sprinkle over gratin. Bake 35 to 40 minutes.