Squash-Tomato Gratin

Totally made this up on the fly with on-hand ingredients tonight—and it was delicious! We’ll definitely have this again and again this summer.
3 summer squash, thinly sliced
1 sweet onion, thinly sliced
1/4 cup melted butter, divided
Salt and pepper
Herbs de Provence
1 large tomato, thinly sliced
Fresh breadcrumbs
Freshly grated Parmesan cheese

1. Place half of squash in bottom of lightly greased baking dish; top with onion rings. Drizzle 1 Tbsp. butter over the top; season with salt and pepper. Place remaining squash over top of onions. Place tomato slices over squash. Drizzle 1 Tbsp. butter over top of tomatoes; season with salt and pepper. Top with breadcrumbs; drizzle with remaining butter. Sprinkle 2 Tbsp. Parmesan cheese over top of breadcrumbs. Cover loosely with foil.
2. Bake at 400° for 30 minutes. Uncover and bake 10 more minutes or until breadcrumbs are golden.