Stir together the yeast and 1/2 cup of warm water in the large bowl of an electric stand mixer.
Set aside and let stand for 5-10 min until the yeast is bubbly.
To the yeast mixture add the remaining 3 cups of warm water, salt and olive oil.
Slowly add the flour one cup at a time and mix with the dough hook until the dough is soft and not too sticky. **You may not need all of the flour.**.
Knead by hand or mix on medium speed for 10 min, or until the dough is no longer sticky.
Set aside to rise for 1 hour or until dough has doubled in size.
Preheat the oven to 425º or grill.
Start the spinach filling while dough is rising.
Saute the garlic in some olive oil in a large saute pan.
Add the spinach leaves and saute until wilted.
Take off the heat and add the mint and toasted pine nuts.
Salt and add red pepper flakes to taste.
When slightly cooled, fold in the crumbled feta.
Punch down the dough, then shape into a rectangle in a well oiled half sheet baking pan.
Dimple with your finger tips and brush with olive oil.
Set aside for 15 min or so.
Place the dough filled sheet pan on the rack of a grill or in the oven and cover with the lid for a few min, just long enough to have the dough slightly rise and firm up - about 5 min or so.
Spread the pesto lengthwise across one half of the dough, leaving a one inch border.
Spread the spinach mixture over the pesto.
Cover with the grill lid and cook for a few more min, until the dough is firm enough for you to lift up the non-pesto half and fold over the filled half - 3-5 minute.
Press the edges together with a fork to keep the filling inside while cooking.
Brush with olive oil.
Move the pan to the side of the grill and continue to bake over indirect heat until it is golden and done - about 7-10 minute.
Remove from heat and let cool for 10 min or so before cutting and serving.