For the dough:
1 medium size russet potato
2 tbsp olive oil
3/4 cup cold water
1 tsp salt
1 tsp baking powder
3 cups all purpose flour
For the fillings:
1 large yellow onion, finely chopped
4 tbsp olive oil
3 large russet potatoes
salt and pepper to taste
1 cup fresh spinach
2 large sweet potatoes
Preheat oven to 350F. Prick the russet potatoes with a fork several times and wrap in foil and place in oven for about 70-80 minutes. The sweet potatoes can go in just as is for about 50 minutes.
To make the dough:
In a large bowl, place one peeled baked potato for the dough, let the others continue to cool. Add the oil and water to the potato and mash until well-combined. It will look like a goopy mess or paper mache but that's how it is suppose to look. Add the salt and baking powder and then add the flour in batches, kneading with each addition. Knead until you have a smooth dough. This can take up to 15 minutes. Let the dough rest while you prepare the fillings.
For the potato knishes:
Saute the onions in 2 tbsp of olive oil over medium heat for 15 minutes until browned and slightly caramelized.
Mash the remaining peeled baked russet potatoes in a large bowl. Add the cooked onions, the remaining olive oil and salt and pepper to taste. Mash together well.
For the spinach knishes:
Saute spinach in a little bit of olive oil for about 5 minutes and until cooked. Add spinach to half of the potato mixture and mix until well combined.
For sweet potato knishes:
Mash the peeled baked sweet potatoes. Add cinnamon, nutmeg and salt to taste, and mix well.
To assemble and bake knishes:
Cut the dough into three equal portions. Roll one portion out as thinly as possible into a 14x6 inch rectangle. It won't be a perfect rectangle, more of an oval, and that is ok. Sprinkle flour as you roll to keep the dough from sticking.
Place 2 1/2 cups of a single filling down the center of the rolled out portion of dough. Spread out so that the filling is roughly 12x2 inches. Fold the dough over the filling the long way to create a roll. Trim the ends of the roll to where the filling begins. Place folded side down on a lightly greased baking sheet.
Repeat with the other two portions of dough and the other two fillings. Don't overstuff! We had extra filling so we threw some in some Pillsbury crescent rolls and they were a really tasty!
With a knife, score each roll into fifths. That will give them a nice shape and make them easier to cut when they are done. Brush each lightly with olive oil. Bake at 350F for 40-50 minutes.
Remove from oven; let cool just enough so that you can slice them. Serve warm and with plenty of mustard for the potato and spinach ones.