Warm 1/4 cup of the reserved spinach liquid to just above 100F (105 to 115, ideally).
Stir the yeast and sugar into the warm spinach liquid, let stand until foamy, 10 minutes.
Add remaining 1/2 cup spinach liquid, feta cheese, oil, spinach, and nutmeg and stir thoroughly.
Mix the flour and salt and stir into the spinach mixture.
Knead on a floured surface or in a Mixmaster until dough is well marbled, 8 to 10 minutes. Add more flour, if the dough is sticky, but it is okay if it is slightly moist. Place the dough in an oiled bowl, cover tightly with plastic wrap, and let rise until doubled, 1-1/4 to 1-1/2 hours.
The dough may be even wetter after rising because of the spinach. Pour the dough onto a well-floured work surface and flour the top of the dough if necessary. Shape into a loaf and place on a lightly oiled bread pan. Cover with a towel and let rise until doubled, about 45 minutes.
Preheat oven to 425°F Bake for 10 minutes, then reduce the heat to 400F and bake the loaf 30 to 35 minutes.
Cool completely on a rack.