Separate the stems and caps of mushrooms.
Heat oil in a kadai. Add asafoetida, cumin seeds, onion seeds, fennel seeds, mustard seeds, fenugreek seeds and sauté on low heat.
Turn off the heat, add ginger-garlic paste and mix lightly.
Turn on the heat again, add mushroom caps and stems and mix. Cook on high heat.
Add red chilli powder, turmeric powder, black salt, salt and toss. Add vinegar and mix. Cook till almost dry.
Transfer into a serving bowl and garnish with cucumber slices. Serve hot.