To make the dough: In a medium bowl, stir together the flour, sugar, and salt. Pour in the oil and lukewarm water and knead lightly until a sticky dough is formed. Set aside, covered, for at least 1 hour while you prepare the filling.

To make the mashed potato filling: Preheat the oven to 400°F. Trim about 1 inch off the head of garlic, exposing the cloves. Drizzle the olive oil on the garlic skins and rub it in. Wrap the head of garlic in aluminum foil and place on a baking sheet in the oven. Roast for 30 to 40 minutes, or until the individual cloves are completely soft.

In a medium saucepan, heat 2 tablespoons of the schmaltz or oil over medium heat. Add the onions and 2 teaspoons of the salt and sauté until golden, 5 to 7 minutes, then remove from the heat and set aside.

Bring a pot of salted water to a boil. Add the potatoes, reduce the heat to medium, and simmer until the potatoes are very tender and can be easily pierced with a fork, about 20 minutes. Drain the potatoes, then roughly mash with a fork or the bottom of a jar. Stir in the onions, remaining 2 tablespoons of schmaltz or oil, remaining 2 teaspoons of salt, and the pepper and squeeze the roasted garlic cloves from their skins into the potato mixture. Stir to incorporate and break up any large chunks of potato. Taste and adjust the salt levels to your preference.

To assemble the knishes: Preheat the oven to 400°F with a rack set in the center. Line a baking sheet with parchment paper.

Divide the dough into 3 balls and the filling into 3 equal portions of about 3 cup each. Roll out the dough on a floured surface, one ball at a time, into a very thin square, about 10 inches on all sides. Place a 2-inch-wide line of filling along one end of the dough, leaving a border of about 11 inches along the edge. Pick up the top edge and fold it over the filling. Brush schmaltz on the dough in a thin strip on the bottom edge of the filling. Pick up the filled dough and roll again, onto the oiled dough. Brush another line of schmaltz at the bottom edge of the filling and fold the filled dough over the oiled dough once again. Repeat the folding and brushing until you reach the end of the dough. Tuck the ends underneath the log.

Place the knishes on the prepared baking sheet, seamside down. Slash the top of the dough rolls with a knife every 3 inches or so. Coat the rolls with egg. Bake for 15 to 20 minutes. Keep an eye on the knishes while they bake to avoid overbrowning and drying out. The knishes are ready when they are soft and golden. Serve warm with mustard.