Heat oven to 400°F. Line a large baking sheet with parchment.
Stir Gruyère, Parmesan, herbes de Provence, garlic powder, red pepper flakes, pepper, and salt in a bowl.
Unfold puff pastry on a lightly floured surface. Roll into a thin rectangle, about 10×14 inches.
Spread Dijon in a thin, even layer over the pastry, leaving a 1/2-inch border on one long edge.
Scatter the cheese mixture evenly over the mustard. Press lightly to help it stick.
Roll up tightly from the opposite long edge into a log. Seal the seam by pinching gently.
Chill the log for 15 minutes to firm for cleaner slices.
Whisk beaten egg with water. Slice the log into 1/2-inch rounds using a sharp knife.
Arrange slices cut-side up on the sheet. Brush tops with egg wash and sprinkle with sesame seeds.
Bake 16–20 minutes until puffed and deep golden. Rotate the pan once halfway if needed.
Cool 5 minutes. Scatter with chives and serve warm.