Mushroom and winter squash gratin

Mushroom and winter squash gratin (Eric Boyd / LAT)

Total time: 1 hour, 40 minutes

Servings: 8 to 10

Note: In October, Russ Parsons shared a quick-searing technique for cooking mushrooms that makes even domestic mushrooms taste wild and woodsy. This standout recipe is drawn from that article.

1 (3 1/2- to 4-pound)

spaghetti squash

1 1/4pounds mixed

mushrooms (portabello, cremini, white button)

4 tablespoons plus 1 1/2

teaspoons butter, divided

2 3/4teaspoons salt, divided

2 tablespoons minced

shallots

3 slices prosciutto, cut in thin slivers

2 leeks

1 cup crème fraîche

1cup cream

1/2round loaf (about 4 ounces) day-old

sourdough

1/3cup (1 ounce) freshly