Preheat the oven to 180 ° C. Cut the onion into half rings, slice the cahmpignons.
Heat the olive oil in a large pan and fry the onion half rings until golden. Add mushrooms and lightly salt. Steam for approx. 6-7 minutes. If there is still liquid, pour it off and pour the contents of the pan into a casserole dish.
Mix the tahini with the water, lemon juice, garlic and salt to a homogeneous cream. It shouldn't be too runny!
Spread the tahini cream over the mushrooms and put the baking dish in the oven. Bake for about 15 minutes at 180 ° C until the tahini turns a light golden color.
Take the pan out of the oven, sprinkle with chopped parsley and pitted, finely diced tomato.