Boil the mushrooms in1/4 cup water or microwave on high for 4 minutes, till soft. Drain and cool. Grind this in a small jar into a coarse paste. You could also use a ricer to mash this. Keep aside.
In a non stick pan, heat 1 tsp oil. Sizzle the Nigella seeds in oil for a few seconds, add the chopped garlic. Saute for a brief while. Do not brown. Put it in the chopped onions with a pinch of salt and saute until soft and lightly browned. This will take around 3-4 minutes.
Next, add the coarsely ground mushrooms and stir for 3-4 minutes on high heat, till the mixture is dry. Remove into a bowl and cool for 5 minutes or so.
In the same bowl, add the smashed potatoes, remaining ingredients and season with salt to taste. Taste the mixture for salt and spices.
Spread the rava/semolina on a flat dish. Shape the mixture for the tikkis into thick rounds. I made 4 large tikkis from the above mentioned quantities. Handling them gently, coat on both sides with rava and shallow fry on a greased non-stick pan or grill for 3 minutes on each side till brown and crisp.
Serve in a burger bun or as it is with ketchup or chutney.