Aromatic Vegetable Turnovers with Yogurt Pastry

For the filling:
1 tbsp butter
2 tbsp olive oil
8 ounces onion, finely chopped (red onion preferred)
1 garlic clove, finely chopped
6 ounces potatoes, cut in small cubes
2 tsp ground coriander
1 tsp ground cumin
1 cup frozen small peas
Handful fresh mint leaves, finely chopped
Salt and black pepper

For the Pastry:
1 stick butter
1 tbsp olive oil
1/2 cup plain yogurt
2 1/2 cups all-purpose flour
1/4 tsp baking soda
1 tsp salt
1 egg yolk

Make the filling first. Melt the butter and olive oil in a large skillet over a medium heat. Add the onions and garlic, and cook for a few minutes, stirring occasionally until the onions begin to soften. Add the cubed potatoes
and cook for 5-7 minutes, stirring occasionally, until they too are just soft. Stir in the spices and cook for one minute; then remove the skillet from the heat. Add the peas.

Turn the mixture into a mixing bowl and season it well with salt and pepper. Stir the mint into the mixture. Leave the mixture on one side until cold.

Lightly grease a large baking tray. To make the pastry, melt the butter and then leave it to cool. Whisk together the olive oil and egg, stir in the yogurt, then, using a wooden spoon, stir the melted butter into the yogurt mixture, a little at a time, until smoothly mixed. Sift the flour, baking soda and salt together. Gradually stir the flour into the yogurt mixture to form a soft dough. Turn out on to a lightly floured surface and knead gently for a few minutes. Then roll the dough out to about 1/4-inch thick. Using the rim of a glass about four inches in diameter, cut out circles from the pastry. Spoon about two teaspoons of the filling on to one side of each circle.
Fold the pastry over to firm a semi-circular turnover. Press the edges to seal. Place on the baking tray and brush with a little egg yolk. Bake in a preheated oven, 400F, for
about 20 minutes or until golden.

-Josceline Dimbleby , The Practically Vegetarian Cookbook