2-3tomatoessliced 1/4 inch thick (about 1 1/2 cups sliced tomatoes)
kosher salt and pepper
fresh parsley leaveschopped, for garnish
grated Parmesan cheesefor garnish
Make the dough:
In a medium bowl, stir the flour and salt together with a fork or a whisk. Grate the butter with a box grater and add to the flour. Using your fingers, begin to work the butter into the dough until you have coarse crumbs. Stir in the water a tablespoon at a time, using a fork to incorporate it into the dough, checking it with your fingers by pinching the dough together. You can also do this with a food processor, if you wish. Do not over work the dough – you want to be able to see bits of butter within the dough, and it will still be rather shaggy and loose. Turn the dough out onto a lightly floured surface and use your hands to bring it together into a ball, but be careful not to overwork the dough. When the mixture pulls together into a ball, wrap the dough in a piece of plastic wrap, form it into a disk, and refrigerate for at least an hour or overnight.
Make the galette:
Preheat the oven to 400°F.
Roll the dough into a 10-inch disk about 1/4-inch thick on a sheet of parchment paper. Return to the refrigerator while you make the filling.
In a small pan, heat the olive oil over medium-low heat. Cook the onions, season with a pinch of salt and pepper, stirring occasionally, until soft and translucent. Set aside to cool.
In a small bowl, use a fork to cream the ricotta, Parmesan and feta cheese. Stir in the cooked onions and salt. Spread in the center of the dough, leaving a 2-inch perimeter. Arrange the tomatoes on top of the cheese, then fold the edge of the dough up on the tomatoes, pleating as you make your way around the galette. Lightly drizzle tomatoes with a touch of olive oil and season with salt and pepper.
Whisk together the egg and water. Lightly brush the dough with the egg wash. Transfer the galette with the parchment paper onto a heavy baking sheet.
Bake in the oven for about 30 minutes, or until the crust is golden brown. Remove from oven and sprinkle with fresh parsley and more grated Parmesan cheese. Let it sit for about 5 minutes before slicing. Enjoy while warm.