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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.
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Steam the spinach until wilted, about 5 – 7 minutes. Remove and wrap it in a tea towel, squeezing it to remove excess moisture.
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In a mixing bowl, stir together the flour and baking soda. Mix in the onions and half the feta.
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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.
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Transfer this mixture into the bowl with the flour and mix with a spatula to combine.
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Preheat oven to 350 F / 170F.
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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.