Cover pot and cook until mussels just begin to open, about 5 minutes.

4 1/4 cups all-purpose flour, plus more for dusting.2 1/2 teaspoons table salt.

1 stick unsalted butter, cut into tablespoons, plus 6 tablespoons melted butter.Canola oil, for greasing.In a small saucepan, combine the milk, squash puree and sugar and cook over low heat, whisking, until the milk is lukewarm and the sugar is dissolved.

Remove the pan from the heat.Whisk in the yeast and egg; let stand until foamy, 10 minutes..

In the bowl of a stand mixer fitted with the paddle, beat the 4 1/4 cups of flour and the salt at low speed just to combine.

With the machine on, slowly add the warm milk mixture and beat until incorporated, about 1 minute.Switch to the dough hook, add flour, and beat at medium speed until a soft, sticky dough forms, about 5 minutes..

Grease a large bowl with butter.Scrape dough onto a lightly floured surface, and knead a few times.

Form dough into a ball, and transfer to the prepared bowl.Cover with plastic wrap, and let stand in a warm place until nearly doubled in size, about 2 hours.. Make the filling:.