Place the flour, cheese, pepper, and salt in a bowl. Whisk to combine.
Cut the butter into the flour mixture using a pastry cutter until it resembles a coarse meal. Stir in the water and lightly knead until the pastry comes together but don’t overwork the dough.
Flatten the pastry into a disc and wrap it tightly in plastic wrap. Transfer the dough to the fridge and chill for 1 hour.
While the dough is chilling, make the filling. Pour the olive oil into a large skillet. Add the onions and a generous pinch of salt. Sauté until just translucent. Reduce the heat to low and cover the pan. Let cook for 20 minutes, stirring halfway through.
Add the garlic to the onions and return the lid to the skillet. Let cook for 5 minutes more. Add the vinegar and sugar and scrape up the brown bits at the bottom of the pan. Take the skillet off of the heat and set aside to cool.
On a floured surface, roll the pastry out to a quarter-inch thickness. Using a two-and-a-half-inch biscuit cutter, punch out a series of circles.
Working with a circle at a time, expand the circle slightly with your rolling pin. Transfer it to a large baking sheet lined with a silicon mat. Spread a little Dijon mustard in the center of the pastry circle and top with the onion filling. Fold the edges of the pastry towards the center to form your galette. Repeat with the remaining pastry.
Transfer the galettes to the freezer and preheat the oven to 375°F. Once the oven is up, place the egg and water in a bowl. Beat to combine and brush the mixture all over the galettes. Bake the galettes for 20-25 minutes or until golden. Transfer to a cooling rack and let cool completely.
When ready to serve, beat the goat cheese and honey together. Transfer the mixture to a small piping bag and pipe it onto the galettes. Garnish with chives and lemon zest and serve.