Pulse the mushrooms in a food processor until they are the size of lentils. Heat the butter and oil, and sauté three-quarters of the red onions with cumin seeds for 10 minutes.

Add the garlic, ginger, and chiles for 3 minutes, then add the tomatoes. Cook for 10 minutes until you have a thick, paste-like base.

Stir in the mushrooms; they will shrink significantly as they cook. After 5 minutes, add the dry spices and salt. Cook for another 5 minutes until the mixture is savory and fragrant.

Stir in the peas and heat through. Spoon the mixture into hollowed-out, toasted brioche buns and garnish with fresh cilantro and the remaining raw red onion.