Combine flour and brewer’s yeast in bowl of food processor; pulse to combine. Add 4 ounces cold butter; pulse until mixture resembles coarse crumbs. Add water, 1 tablespoon at a time, until dough holds together. Form into disk; wrap in plastic food wrap. Chill 2 hours or until firm.
Heat 2 ounces butter and olive oil in large skillet over medium heat. Add fennel, potatoes, garlic, shallots, sage and pepper; cover. Cook 5 minutes. Add mushrooms; cook, covered, 10 minutes more, stirring halfway through.
Add sherry and stock. Cook, uncovered, until almost all of liquid is evaporated. Stir in Extra Melt Monterey Jack Cheese Sauce; simmer 2 minutes.
Pour filling into 10-inch cast iron skillet or serving dish of choice. Cool to room temperature.
Meanwhile, roll out dough on lightly floured surface to 1/8-inch thickness. Top cooled filling with crust. Trim to fit pan, leaving 1-inch overhang. Press dough firmly around edge of pan to seal. Cut vents in crust. Brush with beaten egg. Sprinkle with coarse salt, as desired.
Place pan onto sheet tray. Bake at 400°F for 35-40 minutes or until crust is deep golden brown.