Use a large grate of a cheese grater to grate the stick of butter into smaller pieces and place this into the fridge to stay cool.
Meanwhile, toast the walnuts in the oven for 8-10 minutes or until fragrant. Let cool then pulse in a food processor until a fine powder-like nut flour. Combine the walnut flour with the regular flour and add in the salt and cinnamon. Whisk to combine.
Take the butter out of the fridge and mix this into the flour using a mixer or your hands. Quickly and carefully mix together (don't over mix). The result should be pea-sized granules of flour with butter pieces still whole throughout the mixture.
Add the cold water one tablespoon at a time. Turn the dough out onto a floured surface and gently knead the dough just until it comes together into a ball. Wrap with plastic wrap and place in the fridge.
for the fruit
Combine the fresh fruit with the corn starch, lemon juice, and sugar. Let sit while you roll out the dough.
forming the galette
Prehat the oven to 375 degrees. Roll out the dough on a floured surface to a 12 inch diameter. Use your rolling pin to transfer the dough circle to a parchment lined baking sheet. Spoon the fruit out onto the center of the dough (keeping it in a high mound in the center). Fold up the sides onto the fruit then brush the milk onto the dough and sprinkle with coarse sugar.
Bake at 375 for 45-50 minutes or until golden brown and bubbly.
Serve with ice cream or whipped cream.
Almonds or pecans can be substituted for the walnuts or use a all flour crust by adding 1/2 cup more flour instead of the ground nuts to the dough.