Prepare sheet pan with parchment paper and preheat oven to 400F.
Roll out puff pastry to 1/8" thickness.
Spread Dijon mustard over puff pastry, leaving 1" around edges to roll up.
Sprinkle 1 1/2 cups of Gruyere over puff pastry.
Sprinkle bacon on top of Gruyere cheese. Mmmmm bacon.
Spread tomato slices over top of Gruyere, overlapping slightly.
Sprinkle remaining 1/2 cup of Gruyere on top of tomatoes.
Prepare egg wash. Fold up outer edge of puff pastry, cutting slits every inch along edge. Brush with egg wash.
Bake 30 minutes in oven, until puff pastry lightly browned. Remove from oven and serve warm.