Melt 3 T butter in sauce pan. Sautee onions until translucent. Sprinkle flour in mixing thoroughly, then add milk in to make a roux. Remove from heat and add in cheese to melt. Layer the sausage crumbles and summer squash in a 6 cup casserole dish. Pour the cheese mixture over. For the topping, melt the 2 T butter and mix into the Panko bread crumbs. Sprinkle over the top of the squash. Bake at 350 for about 45 minutes.