For the dough: Place 1 1/4 c. all-purpose flour, 1/2 t. salt, and 1/2 t. sugar in the bowl of a mixer fitted with a paddle attachment and mix to combine. Then, add 1 stick of cold, cubed, unsalted butter and run on low-medium speed until the butter is the size of dry lentils. At this point, slowly add 1/8-1/4 c. ice water in a steady stream, just until the dough holds together. It’s better to err on the side of caution in this step- you can always add more water, so don’t be heavy handed when you start to pour.
Form the dough into a flattened round, wrap in plastic and chill for at least one hour. This resting period allows the gluten to relax, which will leave you with tender, flaky dough. The dough can be made 2-3 days ahead of time.
After the hour + has elapsed, remove dough from the fridge and roll it into a circle approximately 9″ in diameter and 1/8″ thick. Poke holes in the dough with a fork. Chill the dough again, covered in plastic, while the onions cook.
For the filling: Place 2 onions, thinly sliced, into a saute pan with about 1 T. olive oil. Cook them slowly over low heat, stirring often, until caramelized. If the onions start to look like they are burning in places, add a little more oil to hydrate them. While the onions cook, slice 2 small heirloom tomatoes into ¼” slices; set on paper towels to drain. Slice a handful of cherry tomatoes in half and scoop out their seeds. Once done, set the caramelized onions aside.
Set the oven to 350F. While the onions cool, brush a thin layer of egg wash or water around the perimeter of the dough. Fold the edges over slightly, about 1/4″, and seal to the egg wash. Brush more egg wash around the perimeter and use your fingers to pinch the dough, forming pleats or ridges. The beauty of galettes is that they are free-form and meant to look rustic, so don’t stress if the ridges don’t look perfect- they aren’t supposed to be.
Spread the cooled onions over the dough and top with the prepared tomatoes, 2 oz. bleu cheese, and 2 T. chopped walnuts (optional). Season with fresh herbs, salt, and pepper to taste.
Bake the galette for 10-15 minutes, or until the crust is golden brown.