Place the boneless beef roast into a slow cooker. Season with salt and pepper. Add a couple tablespoons of jalapeno pepper juice to the roast. Turn on low to medium heat and let the roast cook until it is tender and shredable. Remove the roast from the slow cooker and shred into pieces. Add a little of the roast jucies back to the shredded roast. Add the salsa, garlic powder, salt, and ground pepper to the roast. Add jarred and sliced jalapeno peppers to the meat and mix. Set aside.
Shred the monterey jack cheese and chop the cilantro and red onions.
Heat a griddle to medium heat and place the flour tortillas on the surface. Add some cheese in the center of each tortilla. Add some of the shredded meat. Cook until the cheese is melted. Remove the tacos and garnish with the cilantro, red onions, and jalapeno peppers.