Place the sweet potato in a steamer basket with an inch or so of water. Cover and steam until very soft, about 20 – 30 minutes. Remove, cool slightly, and peel off the skin. Mash the pulp with a fork to make a cup of puree.

Steam the spinach until wilted, about 5 – 7 minutes. Remove and wrap it in a tea towel, squeezing it to remove excess moisture.

In a mixing bowl, stir together the flour and baking soda. Mix in the onions and half the feta.

In another, clean bowl, whisk the eggs, olive oil, yogurt, and thyme. Whisk the sweet potato puree and stir in the spinach with a spoon or spatula. Season with salt and pepper.

Transfer this mixture into the bowl with the flour and mix with a spatula to combine.

Preheat oven to 350 F / 170F.

Brush a 12-hole muffin tin with Greek extra virgin olive oil. Divide the mixture in the muffin tin, sprinkle with the rest of the feta cheese and bake for 25-30 minutes, or until the muffins are golden and puffed. Remove, invert and remove the muffins and cool on a rack.