For the crust: 

1. Cut the butter into small cubes.
2. Add flour, sugar, and salt in a large bowl, then add the butter cubes and toss to coat.
3. Mix all the ingredients together until it makes a crumbly texture. I used a pastry cutter here, but using your hands works great too. 
4. Add cold water and mix until dough forms completely, kneading it with your hands so it all sticks together. I usually put water in a cup with ice cubes to get it real cold. Add a tablespoon of water at a time until the texture feels right. I usually use about 2 ½ tablespoons of water.
5. Form the dough into a ball or small disk, wrap in plastic and put in the freezer for about 10 minutes before rolling it out.

For the filling:

1. Cut the tip and the dark green part from the leek. You’ll just be using the light green and white part for this. Slice the leek in half lengthwise then cut into ¼” slices. Transfer leeks to a bowl of water to rinse off the dirt. Leeks can be very dirty and it’s important to rinse them in water. Run them through a strainer and hose them with your sink sprayer. 
2. Remove all the skin from the rotisserie chicken and pull all the meat off. My video shows this well. Chop the chicken into chunks that are around the same size. I used a small chicken for this - the one that's $4.99-$5.99.
3. Remove the leaves from the sprigs of thyme and chop into smaller bits. The leaves are already pretty small so this shouldn’t take long.
4. Remove about 5-6 leaves from the sage, stack them, roll them and dice them small.
5. Shred the gruyere cheese.
6. Beat one large egg with a splash of water. You’ll brush this on the dough before cooking the galette.
7. Roll the dough out onto the plastic it was wrapped in. I’ve found this is easier than risking the dough sticking to your cutting board.
8. Add flour to the top of the dough disk and to your rolling pin as needed throughout the rolling process. Try to roll in different directions so the dough rolls out into a large circle. It won’t be perfect and that is okay. Ultimately you want the dough rolled out to about 13”x13” wide.
9. Once the dough is rolled out, flip it onto a parchment-paper lined sheet pan and gently peel of the plastic wrap.
10. Toss the leeks with about a tablespoon of olive oil as well as the salt and pepper. 
11. Transfer the chopped chicken to the rolled-out galette dough and sprinkle a little of the sage and thyme on top of the chicken.
12. Save a tablespoon of leeks off to the side. Add the rest of the leeks on top of the chicken with another sprinkle of the sage and thyme.
13. Add the shredded gruyere cheese on top of that.
14. Start folding the dough over the sides and overlapping with each fold. My video shows this well. If one side is short a little dough, just pull a piece from the other side and patch where needed. Pinch the dough a little as you are folding to hold it together. This is not going to be perfect either and it’s fine. This is a beautiful, rustic pie – don’t worry about perfection!!
15. Once all the sides are folded, slowly pour one cup (slightly more if you think it will fit) of heavy cream through the top opening. 
16. Add the extra tablespoon of leeks to the top of the cheese and a little fresh pepper if you like. 

Brush and blot the dough with the egg wash.

Bake the galette in a preheated oven at 375 degrees for 50-60 minutes until crust is golden brown. Let it cool for 15-20 minutes before serving.