Preheat oven to 400F.
Evenly divide the dough into 3-4 pieces. Use your hands to round out each piece into a ball, then roll out to be 1/8" thick (circular-shaped).
Slice the tomatoes according to your liking, then dip each tomato slice in the dijon mustard mixture so both sides are evenly coated. Place a few slices onto the center area of each rolled-out dough leaving 1" border on the edge. Evenly toss on the spring onions over the tomatoes for each galette.
Fold the rim of the dough up then over the edge of the tomatoes. Overlap the dough as you make your way around so that the dough is pleating. Gently press down on the dough to ensure it stays.
Brush the galette crust with an egg wash (one whisked egg) or olive oil, then (optional) garnish with 'everything but the bagel' seasoning.
Bake for 12-15 minutes, until the crust is golden brown. Allow it to cool for at least 10 minutes. Enjoy warm :)