Kasha Knishes

Name: Kasha Knishes
Cuisine: European, Eastern European
Ingredients: 1 ½ cups dry whole kasha groats (toasted buckwheat); 1 ½ medium yellow onion, diced; 2 teaspoons olive oil; 2 tablespoons butter; 2 Yukon Gold potatoes, peeled and cut into 1 inch pieces; 360g (2 ¾ cups) all-purpose flour; 1 (6g) teaspoon salt; 1 teaspoon (5g) baking powder; 1 ½ (7ml) teaspoon apple cider vinegar; 1 large egg, beaten; 118 ml (½ cup) canola or your preferred neutral oil; 118 ml (½ cup) tepid water; 1 egg, lightly beaten for egg wash