Preheat your oven to 350°F / 175°C. Clean any soil from the mushrooms with a paper towel. Pulse them in batches in a food processor until they are chopped to the size of lentils.
Set aside.
Place a large, lidded frying pan over medium heat.
Add the butter and oil.
Once hot, add the crushed cumin seeds and about three-quarters of the diced red onions.
Sauté for 10 minutes until softened and fragrant.
Stir in the crushed garlic, grated ginger, and green chiles, and cook for another 3 minutes.
Add the chopped tomatoes, stir, and cover the pan with the lid.
Cook the tomato mixture for about 10 minutes, or until the liquid has evaporated and it forms a thick, paste-like consistency.
Add all the chopped mushrooms to the pan. They will seem like a lot, but will cook down significantly.
Stir gently to combine with the paste and cook for 5 minutes.
Add the garam masala, salt, black pepper, and turmeric.
Continue to cook for another 5 minutes, stirring occasionally.
Stir in the peas and cook just until they are heated through.
Taste and adjust the salt if needed, then remove the pan from the heat.
Place the brioche buns in the preheated oven for 5 minutes until warm and slightly crispy. To serve, slice the top off each bun and gently hollow out some of the soft bread from the inside. Spoon the hot mushroom kheema into the hollowed-out buns, sprinkle generously with fresh cilantro and the remaining raw red onion, place the tops back on, and serve immediately.
Adjust the heat to your liking by increasing or decreasing the number of green chiles. For a milder version, be sure to remove the seeds before chopping.