In large bowl mix together Greek yogurt, milk, salt and oil. In a separate bowl, mix together baking powder with flour, then add to the above mixture and knead into a smooth dough.
Roll out dough. Cut into 12 triangles. Put filling on to each triangle. Starting from the wide end, roll up each triangle into a croissant.
Place croissants on to baking sheet and brush with milk. Bake at 400 F (200C) for 20 minutes. Enjoy.