• Preheat the oven to 350°F. Line a baking sheet with parchment paper.

Make the Dough:

• Add the mashed potato to a mixing bowl, add the olive oil and 6 tablespoons of water and mash until well combined. Add the salt and baking powder, mix well.
• Then add the flour, mix and knead until a smooth dough is formed. Add more water, if the dough seems too dry. This might take about 8~10 minutes. Cover and let the dough rest while preparing the filling.

Make the Filling:

• Heat 1tbsp olive oil, in a pan, add the onions and cook till the onions are lightly browned around the edges. Add the garlic and cook for 1 minute.
• Next add the spinach and cook till wilted about 3~4 minutes.
• Stir in the mashed potatoes and mix well to combine. Season with salt and pepper. Let the mixture cool slightly.

Make Knish:

• Divide the dough into 2 equal pieces. Roll one piece into a 10" square on a floured surface, then cut into four 5" squares.
• Scoop about ½cup of filling into the center of each square. Pull dough edges up and around the filling and pinch to close. Place them seam side down on the prepared baking sheet.
• Repeat with the remaining dough. Brush knishes with oil and sprinkle with coarse salt (if desired). Bake for 30~40 minutes or until golden brown.