How to Make It
In glass measuring cup, combine the yeast, sugar, and water and let sit for 10 minutes. In the meantime, mix the flour, baking soda, and baking powder in large bowl.
Whisk together the yeast mixture, yogurt, and milk and stir into the dry ingredients. Knead with your hands until the dough forms a smooth, soft ball, about 2 minutes. Place the dough in an oiled bowl and cover with plastic wrap. Leave at room temperature until doubled in size, about 2 hours.
Sauté the onion, garlic, ginger, and coriander in butter over moderately low heat for 5 minutes. Add the chopped spinach and cook, stirring often, for five minutes, until the spinach has wilted and moisture has evaporated. Stir in the yogurt and let cool for five minutes. Transfer to a food processor and pulse until smooth. Season with salt and pepper and set aside.
Preheat the oven to 400º. Toss the potatoes with oil, season with salt and pepper, and place in one layer on a baking sheet. Roast for 20 minutes, until tender and slightly brown. Set aside.
Remove the dough from the bowl and turn onto a well-floured surface. Knead briefly and divide the dough into 12 equal portions. Roll into 1/4 inch thick ovals, approximately 6 inches wide.
Heat a cast-iron skillet over moderately high heat, and preheat the broiler. Brush the dough with melted butter and place in the skillet. Cook for about one minute, until the dough puffs up. Flip the bread, cover the pan, and cook for one more minute. Remove from the pan and brush both sides with butter and sprinkle with salt. Spread each piece of naan with layer of spinach puree and sprinkle with the roasted potatoes. Broil in the oven for 2 minutes, until edges of the naan turn golden brown.