Make the filing: Place a large skillet over medium heat. Add the olive oil and crushed garlic. Fry the garlic until just browned, about 3 minutes and add the chili flakes and enough spinach to fit the pan. Stir and cover with a lid. Stir occasionally until the spinach is wilted and cooked, about 5 minutes. Add the remaining spinach leaves into the skillet and repeat until all the spinach leaves are cooked. Transfer the spinach mixture off the heat and let it cool until it can be easily handled. Place the spinach mixture into a clean kitchen towel and squeeze as much liquid out of the mixture as possible. Finely chop the drained spinach and place it into a large mixing bowl. Add salt, pepper and the feta or bulgarian cheese, and mix until evenly combined. Set aside.

Make the dough: Place the flour, salt, and sugar into the bowl of a stand mixer fitted with a dough hook attachment. Mix for a few seconds to combine and continue mixing on low speed. Gradually add the water and oil while mixing, and continue kneading the dough. Add enough water until the dough is smooth and firm but not sticky. Transfer the dough to a floured counter and knead for about 1 minute until it is formed into a unified ball. Divide the dough into 10 equal pieces and roll each piece into a ball. Transfer the dough balls onto a baking sheet that is generously greased with neutral oil. Cover with plastic wrap and transfer into the refrigerator. Let the dough rest for 2 hours.

Preheat the oven to 450 degrees Fahrenheit.

Assemble the bourekas: Transfer the dough from the refrigerator and place the dough balls onto a floured surface. Take one dough ball and roll it out into a rectangle that is as thin as possible. Once the dough is at its thinnest, use the palm of your hands to very gently stretch out the dough to a rectangle that is about 10 inches by 15 inches, waiting a few seconds between each stretch for the dough to relax. Brush the dough with melted butter, make sure to cover the surface all the way to the edges. Take about ⅓ to ½ cup of the filling and place it in a horizontal log shape along onto the closest longest edge of the dough. Roll the dough into a log over itself and place the seam side down. Take one edge of the log and roll it to meet the other edge of the log to create a spiral shaped boureka. Transfer the boureka onto a parchment lined baking sheet. Repeat rolling, filing and shaping the remaining dough balls. 

Once the 10 bourekas are shaped, brush them with the beaten egg yolk, sprinkle a generous pinch of salt on each boureka and liberally sprinkle with sesame seeds.

Transfer to the oven and bake for 15 minutes. After 15 minutes, lower the oven to 350 degrees and continue baking for another 45 minutes until the bourekas are deep golden brown and puffed.

Serve immediately with a side of labneh and chopped salad.