In a large mixing bowl, combine the lukewarm water, milk, and salt. Gradually add most of the flour and begin mixing. Knead the dough, incorporating additional flour as needed, until it becomes smooth, pliable, and non-sticky—this should take about 2 to 3 minutes.

Once the dough reaches the desired consistency, divide it into 10 equal portions. Roll each portion into a ball and place them on a lightly floured surface. Cover the dough balls with a clean, damp kitchen towel or plastic wrap topped with a towel to prevent them from drying out. Let them rest while you prepare the filling.

Finely chop the washed and thoroughly drained spinach. If the spinach retains excess water, gently squeeze it out. Transfer the spinach into a large bowl.

Finely dice the onions and place them in a separate bowl. Add salt and massage the onions by hand until they release their juices. Incorporate the onions into the spinach, followed by the red pepper flakes and black pepper. Mix thoroughly, gently rubbing the spinach with the onion mixture to soften the leaves.

If desired, crumble in the Turkish white cheese or feta, folding it gently into the spinach mixture until well combined.

Lightly flour your work surface and rolling pin. Take one dough ball and roll it out into a thin, even sheet about 1mm thick, rotating the dough frequently to prevent sticking. Traditionally, an "oklava" (thin rolling pin) is used, but a standard rolling pin will work as well.

Place a generous amount of spinach filling over one half of the rolled-out dough. Fold the dough over the filling to form a half-moon shape, pressing the edges firmly to seal.

Preheat a griddle or non-stick skillet over medium heat. Place the filled dough onto the hot surface and cook for about 1 minute, or until the underside sets and small spots of color appear. Flip the gözleme and brush the cooked side lightly with butter or drizzle with oil. After another minute, flip again, brushing the other side. Continue flipping and cooking until both sides have golden brown spots and the dough is cooked through, around 4 to 5 minutes in total.

Repeat with the remaining dough balls and filling. To keep the cooked gözleme soft and moist, stack them in a pan and cover with a lid or a large plate until ready to serve.