Mix cherries, cornstarch, lemon, sugar and salt together in large bowl. Set aside until dough is ready.
Cream the butter and sugar together until light in color (about 5-10 minutes). Add the egg, water and salt. Mix until blended. Add the flour and mix until just incorporated. If the dough seems too wet, add more flour. Divide into two disks, wrap in plastic, and refrigerate at least 30 minutes. You will only need one disk for the galette. The other half can be stored in the freezer.
Preheat oven to 375º.
Roll out the chilled dough on a lightly floured surface until you have a 12-inch circle. Carefully transfer the dough onto a metal cookie sheet. Mound the cherry filling in the center of the dough. Gently fold up the outer three-inches of the dough over the cherries, synching at the overlap and leaving the middle open. Brush the dough with the egg wash. Bake for about 30 minutes, or until the crust is golden and the filling is bubbling. Allow the galette to cool completely at room temperature, before cutting. Serve with dollops of vanilla ice cream or freshly whipped cream. This can also be made in individual portions.