Cook the onions and sage in 1 T butter and 1 T olive oil over low heat until the onions are very soft and brown.
Combine the squash, onions, and garlic, season to taste, then place in a buttered casserole dish. Pour the warm cream into the dish, sprinkle with the breadcrumbs.
Bake at 350 until the top is browned and bubbling, about 40 minutes.