How to Make It
Melt the butter in a large skillet over medium-high heat. Add the onion, squash, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring frequently, until tender, 8 to 12 minutes. Transfer to a medium bowl and stir in the sage; let cool.
Line a baking sheet with parchment. Roll the piecrust on a floured surface into a 12-inch circle; place on the prepared baking sheet. Top the piecrust with the squash and onion mixture, leaving a clear 2-inch border. Fold the edges of the piecrust over and toward the center, overlapping slightly and partially covering the squash. Chill until the dough is firm, at least 20 minutes and up to 1 hour.
Heat oven to 350° F with a rack set in the lowest position. Brush the piecrust with the cream. Bake until the squash is cooked through and the crust is golden, 45 to 55 minutes. Top with the blue cheese.
Meanwhile, whisk together the oil, vinegar, and ¼ teaspoon each salt and pepper in a large bowl. Add the frisée and toss to combine. Serve the galette with the salad.