Preheat oven to 415.
In a large skillet over low heat, add the onion slices. Toss them occasionally, caramelizing them until they’re sugary and brown and sweet, about an hour.
Evenly arrange the onions in the center of the rolled out pie crusts. Top with sliced pears (fanned out), blue cheese, walnut pieces and the prosciutto strips.
Fold the outer two inches of dough into the galette, kind of overlapping.
In a small bowl, lightly beat the egg. Brush the egg wash over the outer crust, and bake the galettes for about 30 minutes on a rimmed baking sheet.
Slice and serve! Garnish with fresh rosemary if desired.
