Place the water and spinach in a saucepan over medium heat. Wilt the spinach, pressing down with the back of a spoon, for about 45 seconds. Drain the spinach well, reserving the liquid. Squeeze the spinach to get as much of the liquid out as possible. (You should have about 1 cup spinach water and 1/2 cup spinach.)
Pour the liquid into a mixing bowl and add 4 tablespoons of the butter. The butter will melt and cool down the liquid to about 110ºF. Add the yeast and stir to dissolve. Beat the egg into the mixture. Add the sugar, salt, and black pepper. Finely chop the spinach and add to the yeast mixture.
Add the flour and mix well with a wooden spoon until the dough comes away from the sides of the bowl. Using your hands, form the dough into a ball. Lightly oil the bowl. Place the dough in the bowl. Turn the dough to oil all sides. Cover with a clean cloth and set aside to rise in a warm, draft-free place until it doubles in size, about 1 hour.
Butter an 8 1/2 X 4 1/2 X 2 1/2-inch loaf pan with the remaining butter.
Invert the dough onto a floured surface and punch it down with your fist. Fold each side in and tuck the ends into the center. Repeat. Put the dough, seam side down, into the pan. With the tip of a sharp knife, make 3 slashes lengthwise down the center of the dough.
Sprinkle the dough with the kosher salt and cheese. Cover and let rise in a warm, draft-free place until it doubles in size, about 1 hour.
Preheat the oven to 375ºF.
Bake for about 40 minutes, or until golden. Remove the pan from the oven and place on a wire rack to cool before removing from the pan.
Slice to serve warm or at room temperature.
Yield: 1 loaf