Cut the buns in half. Switch on the oven with grill function for preheating. Put the bun halves on a dry baking sheet, cut them up, place the baking sheet in a preheated oven, and fry the buns until golden brown for about 2 minutes.
In a small bowl, combine yogurt, mustard, oil, oregano, and salt. Brush bun slices with this sauce and sprinkle with chopped cheese. Put the tomato halves on top of the cheese and sprinkle with onions. Put the baking sheet back in the oven under the grill, cook the hot tomato sandwiches for about 1-2 minutes, until the cheese melts.