Cut the onion, tomato, and bell pepper into pieces similar in size to the mushrooms so they cook evenly on the skewers.
Take a mixing bowl and add mushrooms, chopped onion, tomato, and bell pepper. Add yoghurt, ginger garlic paste, vinegar, butter, all the spices, kasuri methi, and salt. Mix gently until everything is well coated. Cover and refrigerate for about 30 minutes.
You can cook this on charcoal for a deep smoky flavor or on a gas stove over low to medium heat. Charcoal gives a more authentic BBQ taste, but both methods work well.
Thread the mushrooms and vegetables onto skewers, placing onion pieces at the start and end to hold everything in place.
I use tamarind wood charcoal (imli koyla), which is one of the best for BBQ and gives a rich smoky flavor. Place the skewers on a grill over hot charcoal and cook evenly, turning them from time to time so all sides are nicely grilled.
Brush butter while cooking to keep the mushrooms juicy and flavorful.
Once done, remove from heat. Your BBQ grilled mushroom skewers are ready to serve. Add sliced onions or your favorite chutney before serving if you like.