Preheat oven to 400°F (200°C). Line a baking sheet with parchment.
Sauté onion in butter over medium heat until soft, about 5 minutes. Season with salt and pepper. Cool slightly.
Roll crust on the sheet. Spread Dijon in the center, leaving a 1-inch border.
Layer onions, apple slices, and cheddar. Sprinkle thyme, salt, and pepper.
Fold edges over filling, overlapping as needed to form a rustic rim. Brush crust with beaten egg.
Bake 25–30 minutes until crust is golden and cheese bubbles. Cool 5 minutes before slicing.