Heat a nonstick skillet over medium-high heat and add the ground beef. Break up the meat into small pieces and let it brown.
While the meat is browning, place the mushrooms, onion, jalapeno pepper and garlic cloves into the bowl of a food processor, and pulse until the mixture is minced.
Add the minced vegetables to the meat, and continue to cook for about 5 more minutes, stirring to incorporate the meat and vegetables.
Add the chili powder, cumin, salt and pepper and stir to combine.
Add the tomatoes, and reduce the heat to medium-low. Let the liquid cook off for about 5 minutes.
Turn off the heat, and add the scallions and cilantro. Stir to combine.
Spoon the taco mixture into lettuce leaves or tortillas. Top with pico de gallo and sliced avocado if desired.