For the slow cooker: in a 4- or 5-quart slow cooker, add the beef roast. Whisk together the broth, vinegar, salsa, cumin, chili powder, onion powder, garlic powder, salt and pepper. Pour over the roast, cover the slow cooker, and cook on low for 8-10 hours until the beef is tender and shreds easily.
For the Instant Pot/Electric Pressure Cooker: in a 6-quart electric pressure cooker, add the beef roast. Whisk together the broth, vinegar, salsa, cumin, chili powder, onion powder, garlic powder, salt and pepper. Pour over the roast, secure the lid on the pressure cooker and close the valve. Cook on high pressure for 60 minutes. Let the pressure naturally release for 15 minutes (or all the way). Release the remaining pressure.
Remove the roast and place in a large bowl or shallow pan (like a 9X13-inch pan). Shred into bite-size pieces with two forks.
Turn the Instant Pot to sauté and bring the liquid to a simmer (for a slow cooker, pour the liquid into a saucepan and bring to a simmer).
In a small bowl, whisk together the cornstarch (or flour) with about 2-3 tablespoons water (use a bit more water if using flour). Add the slurry to the simmering liquid, whisking constantly, and cook, stirring constantly, until the mixture is thickened slightly, 4-5 minutes. Add additional salt and pepper to taste, if needed.
Pour about 1/2 cup sauce in with the beef and toss. Use as desired (instructions for enchiladas below).
To make enchiladas: Spread about 1/2 cup sauce on the bottom of a 9X13-inch pan.
Toss the cheddar and Monterey Jack cheeses together.
Spread shredded beef in the center of each tortilla. Sprinkle with cheese. Roll up and place in the pan, seam-side down.
Spread 1-2 cups of sauce over the top of the enchiladas. Sprinkle with cheese.
Bake at 350 degrees for 20-30 minutes until bubbly and hot. Let the enchiladas rest for 5-10 minutes before serving.
Sprinkle with fresh cilantro, if desired, and serve.
Serving Size: it kind of depends on how full you stuff each enchilada (and the size of tortillas you use) to determine how many total you'll get from this recipe. I've detailed the recipe for a 9X13-inch pan of enchiladas (about 10). If you used a slightly larger roast or used less meat mixture for each enchilada and have more enchiladas, simply fill up another pan and freeze before baking. Freezable: the cooked, shredded beef freezes really well. Toss the beef with 1-2 cups sauce and freeze (in bags or covered containers). Assembled enchiladas freeze very well, too. To bake from frozen, keep the pan covered and bake at 350 degrees for 45-60 minutes; uncover and bake until heated through and bubbly, another 20-30 minutes.