In a heavy pan, sauté onions in olive oil with a pinch of salt until soft but still textured, remove from heat. Meanwhile slice squashes 1/2 inch thick, grate cheeses and prepare herbs. Combine breadcrumbs and cheese.
Spread onions into the bottom of a 8 x 8 casserole crock. Press yellow squash slices into onions, sprinkle with salt and pepper, 1/2 tsp thyme and half of the cheese & breadcrumb medley, then drizzle with 1/3 cup of the cream. Top with zucchini and repeat. Dot with butter and place in the top of a 350-degree oven until bubbling hot and golden brown, about 40 minutes. Rest 15 minutes before serving.
