Preheat the oven to 350°F. Prick the seven regular potatoes with a fork, wrap in foil, and place in the oven. The four sweet potatoes can just go in as is. Take out the sweet potatoes after 40 minutes; the regular potatoes will need to bake for another 30 minutes, so they will take about 70 minutes total, depending on their size. Remove from the oven and let cool. Once the potatoes are cool enough to handle, remove the skins with a peeler or paring knife.

By hand: In a large bowl, place one peeled, baked russet potato; let the others continue to cool. Add the oil and cold water to the potato and mash until well combined. 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 by hand, so have someone nearby to help knead in case your little hands get tired. Let the dough rest while you prepare the fillings.

Using a mixer: In the work bowl of a standing mixer fitted with a dough hook, place one peeled potato and mash it up a bit with the oil and cold water. Add the salt and baking powder, and then add the flour in batches, mixing after each addition. Let knead until a smooth dough forms, about 15 minutes. Let the dough rest while you prepare the fillings.

In a medium-size skillet, sauté the onion in 2 tablespoons of the olive oil over medium heat for 15 minutes, until browned and slightly caramelized.

Mash the remaining six peeled, baked russet potatoes in a large bowl. Add the cooked onion, the remaining 2 tablespoons of olive oil, and salt and pepper to taste. Mash together well. Reserve half of the mashed potato filling in a separate bowl for the spinach filling.

Add the thawed, drained spinach to the reserved half of the mashed potato filling and mix until well combined.

For the sweet potato knishes:

Mash the baked sweet potatoes. Add the nutmeg, cinnamon, allspice, and salt, and mix well.

Preheat the oven to 350°F.

Cut the dough into three equal portions. Roll out one portion as thinly as possible, into a 14 x 6-inch rectangle. Sprinkle with flour as you roll, to keep the dough unsticky.

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 12 x 2 inches. Fold the dough over the filling the long way to create a roll. Trim the ends of the roll-up 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. You may have extra filling, depending on how big your potatoes were. Don't overstuff the knishes; just bite the bullet and eat the filling on its own.

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 for 40 minutes.

Remove from the oven; let cool just enough so that you can slice them. Serve warm, with plenty of mustard for the potato and spinach ones.