Methods

To make the ground beef filling

1. Heat a large skillet over medium-high heat. Crumble the ground beef into the pan into a single layer. Sprinkle the top of the meat with salt, and cook, undisturbed, until the bottom of the crumbles are deep golden, 5 to 7 minutes. Reduce the heat to medium, add the tomato paste, if using, and taco masala and, using a wooden spoon or spatula, stir to combine and cook, breaking the crumbles into smaller pieces, 1 to 2 minutes. 
2. Stir in the water, reduce the heat to medium-low, cover, and cook, stirring occasionally, until the water has reduced by half and the sauce is clinging to the meat, 8 to 10 minutes. (If the water is cooking off too fast, you can add a couple tablespoons of water at a time to keep things saucy.) Serve hot with tortillas or crunchy taco shells, plus your favorite toppings.

To make the mushroom filling

1. Heat 2 tablespoons neutral oil in a large skillet over medium-high heat. Add the mushrooms and salt, stirring to coat the mushrooms in the oil, cover, reduce the heat to medium, and cook, stirring occasionally until the mushrooms have released all their liquid, 8 to 10 minutes.
2. Return the heat back to medium-high, add the remaining 1 tablespoon neutral oil, and cook, uncovered, stirring occasionally, until most of the mushrooms are caramelized on at least one side, 4 to 8 minutes. Stir in the taco masala and cook for another 30 to 45 seconds, then add the water, reduce the heat to medium-low, and continue to cook, stirring occasionally, until the water has reduced by half and the sauce is clinging to the mushrooms, 4 to 7 minutes. (If the water is cooking off too fast, you can add a couple tablespoons of water at a time to keep things saucy.) Serve hot with tortillas or crunchy taco shells, plus your favorite toppings.