Make the pastry dough: In a bowl of a food processor or using a pastry cutter, stir together the Bisquick and salt. Cut in the butter until you have coarse crumbs. Stir in the egg, then add ice water a tablespoon at a time, until the dough just comes together. Dump the dough onto a board dusted with Bisquick. Form it into a ball, then wrap in plastic wrap and chill about 1 hour, or up to a day in advance.
Preheat the oven to 375° F. Place the rack with a baking stone, if you wish, in the center of the oven.
Heat the olive oil in a skillet. Add the sausages and cook for about 2-3 minutes. Add the onion, sweet potato, and mushrooms and cook until the onions are wilted and the sweet potatoes are just starting to become tender. Stir in the spinach and season with salt and pepper.
Roll out the dough into a circle about 12 inches in diameter. Transfer the round onto a parchment paper. Place the filling in the center, sprinkle with feta cheese. Pull up the sides of the dough and pleat. If you wish, refrigerate the galette 10 minutes.
Transfer the galette with the parchment paper and place on baking stone in oven or on a baking sheet. Bake for about 20-30 minutes, or until the crust is golden brown. Allow the galette to cool slightly before slicing. Serve warm.