For the Filling:
Cut chicken breasts into 1-2 inch sized cubes.
In a frying pan, heat the olive oil and add the chicken pieces.
Fry lightly until the chicken changes color and add then add the paprika, herbs and salt.
Cook until the chicken is cooked through, about 3-4 minutes. Set aside.
Thinly slice the tomatoes. Set aside.
For the Crust:
In a food processor, place the flour, salt and cubed butter. Plus a few times (about 3-4 times) until the mixture resembles coarse, pea-sized crumbs. Transfer to a medium sized bowl.
Add ice cold water to the mixture and stir with a fork. Add up to 2 tablespoons of more water if the dough seems dry.
Gently knead the dough a few times until it comes together. Shape into a ball and place covered in the fridge for at least 1 hour.
For Assembling the Galette:
Preheat oven to 190 C (425 F) and line a baking sheet with parchment paper. Set aside.
Take out the dough from the fridge, and a lightly floured surface, roll out the dough into a 12 inch circle. The shape doesn't have to be perfect. Transfer the rolled out dough to the baking sheet.
Spoon the chicken filling into the centre of the dough, leaving a 2-3 inches border. Spread the shredded mozzarella cheese evenly on top of the chicken. Finally, top the cheese with the thinly sliced tomatoes.
Gently fold the edges of the dough over the chicken and tomato filling, overlapping the dough as necessary. Press gently to seal the edges.
Brush the crust with the egg wash.
Bake in the oven for about 25-35 minutes until the crust is golden brown.