Cut the cheese into cubes, as well as the onion. Put everything in a food processor with a universal chopper, plus the mushrooms, garlic, yoghurt, oil and spices.
Let the food processor chop so that the pieces are small enough to be smeared on the bread (it should stay a little bit chunky, then the mushrooms will come into their own).
If you don`t have a food processor, you can grate the cheese, finely chop the mushrooms, garlic and onion and then mix everything with the rest of the ingredients.