Melt butter in large skillet over medium-low using measuring spoon. Add sliced onions and thyme with wooden spoon. Cook 35-40 minutes stirring often till deep golden soft jammy, no burn.
Preheat oven 400°F. Line baking sheet parchment. Roll puff pastry rectangle 12x10 inches using rolling pin on floured board. Transfer sheet. Spread Dijon even with back spoon. Season salt pepper from measuring spoons.
Scatter caramelized onions over mustard uniform ½-inch edge border. Sprinkle Gruyere cheese full cover. Brush edges egg wash using pastry brush for golden shine.
Bake middle rack 20-25 minutes till pastry puffs deep gold, cheese bubbles brown. Cool rack 10 minutes for clean slice.
Cut 8 wedges with pizza cutter or knife. Serve warm or room temp with greens.
