For the pastry, combine the flour and salt in the bowl of a food processor and quickly cut in the cold butter. This should only take about five or six pulses.
In a small bowl, beat the egg and milk together and evenly pour over the flour and butter mixture. Pulse until a rough dough forms.
Dump the dough onto plastic wrap, press out into a rough rectangle, wrap well, and refrigerate for at least one hour.
When ready to bake, preheat your oven to 375F and remove your dough from the fridge. Allow it to sit at room temperature while you prepare the rest of your galette.
In a small bowl, mix the sour cream, mustards, half of each of the chopped herbs, and a pinch of salt and pepper. Set aside.
Thinly slice your tomatoes, letting any extra juice fall away, and set aside.
On a well floured surface, roll out your dough into a rough rectangle, about 2.5 centimeters wide, 45 centimeters long, and 0.5 centimeters thick. Leaving a border about four centimeters wide on each side, evenly smear the mustard mixture over the centre of the dough.
Sprinkle half of the cheese on top of the mustard mixture, top with tomato slices, and finish with the rest of the cheese, a scattering of the remaining chopped herbs, a drizzle of olive oil, and salt and pepper.
Fold the border of the dough up over the edges of the tomato and cheese filling and brush the outer crust with egg wash.
Bake on a parchment lined baking sheet in a preheated oven for 30 - 35 minutes, being careful that the top does not brown too much. If the top does look like it's going a little over, simply tent a piece of aluminum foil over the top of the galette.
Allow the galette to cool slightly and slice into 12 for appetizers or hors d'oeuvres, or into six for a light meal with a salad.