Heat butter over medium-low heat in sauté pan with lid. Add onions and salt. Cover and cook for 10 minutes, stirring occasionally. Uncover and increase heat to medium. Continue cooking for 20-25 minutes, until onions have turned a dark caramel color. Stir often and add balsamic, sugar and mustard and cook for 1 minute. Remove onions from pan and let cool.
Preheat oven to 375F degrees and place oven rack in center position. Place BONMAT™ on Perforated Baking Sheet and set aside. In a mixing bowl, combine Fontina cheese and thyme. Set aside.
Roll out dough onto ROUL’PAT® into 2 circles, 1/8″ thick. Transfer to BONMAT™ and begin layering ingredients but leaving a 2-inch border. Begin with caramelized onions, sausage, then layer pear slices in a circle. Add Gorgonzola, then Fontina/thyme mixture, then more Gorgonzola. Repeat with second galette.
Fold up border and pleat as you go around, keeping the center exposed. Brush border with egg yolk using Silicone Pastry Brush and season with pepper.
Bake for 35-40 minutes or until crust is golden brown and inside is bubbly. Cool for 8-10 minutes before drizzling balsamic over top.