Mix the Gruyere, sage, oregano, and pepper together in a small bowl.
Lay the puff pastry on a lightly floured surface with the short side closest to you.
Spread the mustard evenly over the sheet of puff pastry, leaving a one inch border at the far edge.
Top with the pepperoni.
Top with the Gruyere mixture.
Brush the far edge with the egg.
Roll the puff pastry tightly from the closest edge toward the egg-coated edge.
Lay the log seam-side down on a baking sheet and chill until firm, about 30 minutes or up to a day.
Pre-heat the oven to 400°F.
Cut the logs into 1/4-inch-thick slices and arrange, cut-side down, 1 inch apart on a parchment-lined baking sheet.
Bake, one baking sheet at a time, in the middle of the oven until golden brown, about 14 minutes.
Serve warm.