To prepare the meat loaf, melt the butter in a medium skillet over medium-high heat. Add the bread crumbs and toast until browned, stirring often, for 3 to 5 minutes. Transfer to a large bowl and set aside to cool.
Place the water next to your cooktop. Heat the oil with the onion and salt in a large pot over medium-high heat, cooking the onion until it’s soft and just starting to brown, stirring often, 4 to 5 minutes. Stir in the ginger and cook, stirring often to prevent the ginger from burning, and splashing with water if it starts to brown too much, until it’s fragrant, about 30 seconds. Add the garlic and jalapeño and cook until the garlic is fragrant, splashing with water if necessary, for about 1 minute. Stir in the coriander, cumin, and ground peppercorns and cook for 1 minute, stirring and scraping the bottom of the skillet often and splashing it with water whenever the spices or onion begin to stick to the bottom of the pan.
Add the mushrooms and cook, stirring often, until they release their liquid and the liquid has evaporated, 6 to 8 minutes. Stir in the cayenne pepper and cook for 30 seconds. Stir in the Garam Masala, turn off the heat, and set the pot aside to cool.
Preheat the oven to 300°F. Add the remaining ingredients to the bowl with the toasted bread crumps, kneading it until everything is completely incorporated. Add the cooled mushroom mixture and knead again until combined. Divide the mixture evenly into two 8½ × 4½-inch loaf pans and bake for 1 hour (or cover the loaf pan with plastic wrap, place inside of a resealable freezer bag, and freeze for up to 3 months; defrost overnight in your refrigerator before baking).
While the meat loaves bake, make the tamarind glaze. In a small bowl whisk the glaze ingredients together. Remove the loaves from the oven and carefully drain off the pooled fat from the pan. Evenly spread some glaze over each meat loaf and continue to bake for an additional 30 minutes, or until the internal temperature reads 165°F on a digital thermometer. Let the loaves cool for 15 minutes before serving. Run a knife around the pan’s edges, slice, and serve.