Preheat the oven to 350°. Butter a shallow 2 ½ quart baking dish. In a vegetable steamer or microwave, steam the squash until tender but not mushy, about 5 min; transfer into a large bowl.
In a small skillet over low heat, cook the onion in 1 ½ T of the butter until soft and translucent, about 7 mins. Add to the squash along with the jack and parmesan cheeses, cream, coriander, and salt and pepper. Spoon the mixture into the baking dish.
Melt the remaining butter. Sprinkle the top of the gratin with the bread crumbs and drizzle with butter. Bake until bubbly and golden brown, about 30 mins.