Ingredients:

For the crust

• 1 1/4 cups all-purpose flour, chilled in the freezer for 30 minutes
• 1/4 teaspoon salt
• 8 tablespoons (1 stick) cold unsalted butter, cut into small cubes
• 1/4 cup sour cream
• 2 teaspoons apple cider vinegar
• 1/4 cup ice water

For the filling

• 2 tbsps sweet butter
• 4 cups diced Vidalia onion (~3 onions)
• 2 tsps minced garlic
• 2 tsps fresh thyme or 1 tsp dried thyme
• 1/2 tsp kosher salt
• 1/2 cracked black peppercorn
• 1/2 cup white wine
• 2 tbsps all purpose flour
• 1/2 cup heavy cream
• 1/2 cup Fontina cheese, shredded
• 1/2 cup Gruyére cheese, shredded
• 1 egg + 1 tbsp water, whisked into an egg wash

Directions:

1. Whisk together the flour and salt in a large bowl. Sprinkle cubes of butter over dough and using a pastry cutter or rounded dough scraper to cut into it until the butter breaks up into lentil-sized pieces. 
2. Whisk together the sour cream, apple cider vinegar and water in a small bowl. Add to the butter-flour mixture. 
3. Gently roll the mixture with your fingers until large lumps form. Pat the lumps into a ball. Do not knead the dough. Wrap the ball in a sheet f plastic and refrigerate for 1 hour.
4. While the dough rests in the refrigerator, melt butter in a large skillet over medium-high flame.  Add the onions, garlic, thyme, and salt & pepper, and  sautée until the onions turn golden brown. 
5. Add 1/2 cup of white wine (I used leftover Chardonnay in the fridge). Turn heat up to high. Stir onions while scraping away any brown fond in the pan until all the wine has reduced. 
6. Reduce heat to low, add in flour and stir quickly until combined. Slowly add heavy cream until thoroughly mixed. Stir until moisture has evaporated and onions look dry.  Remove from heat. Add half of shredded cheeses and allow to cool. 
7. Preheat oven to 375F. 
8. On a floured counter, roll the dough out with a rolling pin into a 12-inch circle. Transfer to parchment paper and place on baking sheet. 
9. Sprinkle remaining half of the shredded cheeses in a circular motion over the rolled-out dough (leaving a roughly 2″ border along the edges), then top with onions. 
10. Fold the dough in pinches (see video) around the edge of the onions, then brush edges with egg wash. 
11. Bake galette for 50 minutes or until the crust is golden and the cream cheese onion mixture is bubbly and brown. 
12. Remove from oven and allow galette to rest for 10-15 minutes before slicing to serve. 

The galette may be placed in a refrigerator overnight until ready to bake after step 10.