Add yeast to warm water ( and sweetener if using ), bloom slightly.
In processor, mix salt with flour ( and oil and lemon if using),
add spinach, blend until uniform. add feta and pulse to combine.
Dump in bowl and add yeast/water mix.
Spatula fold till combined. Cover bowl with plastic wrap.
Let sit on counter 19 hours.
Punch down and fold edge to center 8 times. Let rest for 15 minutes.
Line dutch oven with parchment. Sprinkle with cornmeal.
Sprinkle with flour to reduce stickiness, form ball, place in dutch oven.
Sprinkle with more cornmeal. Cover with lid.
Let rise 2 hours.
Preheat oven to 375.
Optional - stick temp probe in center of loaf.
Bake until internal temp is 206-210 F ( about 45 minutes).
Remove loaf to cooling rack.
Let rest about an hour before cutting.