In a large bowl, whisk together flour, rosemary, thyme, salt and pepper. Using a pastry cutter or with your fingers, cut in butter until mixture is crumbly. Add 6 tablespoons ice water. Stir dough, adding 1 additional tablespoon ice water at a time, until dough just comes together. Transfer to a lightly floured surface and knead until it forms a ball. Wrap in cling film or plastic wrap and refrigerate for 20 minutes.
While dough is chilling, heat olive oil in a medium pan over medium low heat. Add shallots. Cook 5 to 6 minutes, stirring occasionally, until they soften and become translucent. Add garlic. Cook an additional 1 to 2 minutes until fragrant. Add chard. Cook for 5 to 6 minutes, stirring occasionally, until chard wilts and softens. Stir in olives. Remove pan from heat.
Heat oven to 400°F.
Remove dough from refrigerator. On a lightly-floured surface, roll into a large circle about 1/8- to 1/4-inch thick. Spread Dijon mustard in a thin layer in a circle in the center, leaving about 2 inches around the sides. Add 1 cup Gruyère and the chard-olive mixture to the center. Gently fold edges of dough over the chard-olive mixture, pleating as you go, leaving an opening about 3 to 4 inches wide in the center. Brush pastry with beaten egg. Sprinkle the remaining 1/2 cup Gruyère over the top of the galette.
Carefully transfer to a parchment-lined baking sheet. Bake for 40 minutes or until pastry is golden brown around the edges.