**Make one day ahead if time permits

Cut the butter, into 1/4" slices using a knife, or a bench scraper. Pop the butter in the freezer for 5 minutes while you prep the rest of the ingredients.

Measure out the milk and apple cider vinegar and mix together. Let set for 5 minutes to sour (this is your buttermilk).

Mix the flour and salt together in a large bowl. Get the cold butter out of the freezer and drop it into the flour/salt mixture. Use a pastry cutter or a fork to cut the butter into the flour. Stop working the butter into the dough when the butter pieces are about the size of almonds, or slightly smaller.

Make a well in the center of the bowl and add the milk mixture. Use your hands to mix the flour mixture with the milk mixture. Dump the mixture out on a floured surface when the mixture resembles a shaggy mass. Use a bench scraper to gather all the bits of flour together shaping it and pushing it together. Make a well in the center of the dough and add 2 Tbs of cold water. Use the bench scraper to start folding and pressing the dough over and on top of its self, working in the water. Work quickly so the dough stays cold. Fold it over on its self about 9-10 times shaping it into a circular mass as you go, using your hands as needed. Flatten it out using a rolling pin to about 1/2" making sure it is in a rounded shape. Wrap it securely in plastic wrap and refrigerate overnight or for at least two hours.

Grate the zucchini using a microplaner or box grater, reserving 1/2 of one zucchini. Cut the reserved zucchini into 1/8" slices to be arranged on top of the galette before baking. Set aside. Place the shredded zucchini in a colander over the sink and add 1 tsp of salt. Mix well. Let sit for 25 minutes.

Place zucchini, in 1 C batches, on a kitchen towel. Squeeze out the liquid and place the shredded zucchini in a large mixing bowl. Set aside.

While the zucchini is draining, cut the onions in half and slice length wise cutting in thin 1/2 moon shapes. Melt the olive oil and butter in a large non stick sauté pan over medium-low heat. Place the onions in the pan with 1 tsp salt, and sugar. These will take about 30 minutes to caramelize. Stir occasionally.

Remove the pastry from the fridge. Have ready a sheet pan, plastic wrap, parchment paper, flour for dusting, a rolling pin and ruler.

Dust the work surface with about 1 Tbs of flour. Place the pastry on the work surface. Use the rolling pin to 'condition' the pastry by beating it down by about 1/4". Turn the pastry as you work it.

Flour the rolling pin and the top of the pastry to keep them from sticking to each other. Use the rolling pin to start rolling out the dough turning the dough 1/4 way around as you roll it out and the dough gets larger. Keep rolling, adding flour under and on top of the pastry as needed to keep the pastry from sticking. Work quickly, you want it to stay cold! If it's sticking too much, place the pastry on the parchment lined sheet pan and pop it in the fridge for 10 minutes. Roll the pastry out to 16" round. If there is a tear in the edge, tap your finger into a bit of water and seal up the tear.

In the large mixing bowl with the zucchini, add the caramelized onions, ground pepper, red pepper flakes, tarragon, dried thyme and gruyere. Mix well and taste for adjustment in seasoning. Adjust if needed.

Mix the egg and 1 Tbs of water well. Have a pastry brush ready.

Assemble the galette on a parchment lined sheet pan. Spoon the zucchini mixture in the center of the pastry leaving at least 2" of crust on the edges. Begin folding the pastry inward, over the zucchini mixture. The pastry will overlap all the way around.

Lay the 1/8" cut zucchini in a decorative overlapping circle on the open part of the galette.

Brush the egg/water mixture over the pastry.

Bake the galette for 45 minutes on the center rack, turning 1/2 way through the bake time.

Let rest for 5 minutes, then cut using a pizza cutter.

Garnish with fresh thyme leaves and flowers (if available).