Make the yeast starter: Dissolve the yeast, salt, and sugar in the warm water and let stand for about 10 minutes at room temperature, until frothy.

In the bowl of a stand mixer, combine the flour, olive oil, and salt. Add the yeast starter and warm water to the bowl and mix together, adding up to ¼ cup more flour if the dough is too sticky. The dough will be sticky; use oiled hands to gather it.

On a lightly floured surface, knead the dough until smooth. Divide the dough into 20 walnut-sized balls and place into a well-oiled dish, drizzling with oil. Let the dough rest, covered, for one hour, until nearly doubled in size.

Meanwhile, wash and dry the spinach. Finely chop the washed spinach, then mix together with the ½ cup of the Parmesan cheese, the cheddar, and the feta, seasoning with salt and pepper to taste.

Preheat the oven to 350F. Begin to shape: Gently roll out each ball into a rectangular shape, rolling it out as thinly as possible. The dough should be translucent — take care not to tear it while rolling.

Arrange two tablespoons of the spinach filling in a log-like shape atop the dough. Fold in the bottom and sides and roll into a log. Shape into a coil. 

Repeat with the remaining balls of dough, until all the boyos have been shaped and coiled. Place the boyos on a parchment paper lined baking sheet.

Brush each of the boyos lightly with olive oil and sprinkle with the remaining Parmesan. Bake until the tops are lightly browned, and the bottoms crisp, around 25 minutes.