Thaw your roast out until almost thawed. You can thaw it completely, but I like to have it still a little frozen because it's easier to cut. Then use a big knife to cut into four large chunks (you may also have to remove some twine, depending on the roast).
Combine all the seasonings (salt thru chili flakes) in a small bowl, then rub thoroughly over your four roast chunks.
Add the olive oil to the bottom of your instant pot or to a cast iron skillet on medium-high heat. Saute the four seasoned chunks of roast until medium brown on all sides. I fried mine two at a time rather than all four at once because the instant pot is too small for all four.
Once the meat is browned, remove from the pan. Add a bit more olive oil if needed, then add the onions. Saute for a minute until they start to caramelize, then add the pepper and jalapeno. Saute for another 1-2 minutes until lightly brown.
At this point, if you're using the instant pot to saute, turn off heat. If you're using a cast iron pan, transfer all onion mixture to instant pot, then swish about 2 tablespoons of water in cast iron pan to pick up flavors from pan and pour water into instant pot. If you want to use a slow cooker or roaster, add onion mixture to slow cooker/roaster and pour the water in as well.
Add the tomatoes, garlic, cilantro, and the beef roast chunks to the instant pot, slow cooker, or roaster.
Instructions for Instant Pot:
At this point, just put the lid on the instant pot and turn it to the manual setting. Don't add any more liquid. Set the time to 1 hour, and set the valve to "sealing." And that's it! Just let it go for an hour until it beeps. At that point, make sure it's off and then set the valve to "venting." And when you move to venting, make sure your face is away from the instant pot. The "venting" valve will release a lot of very hot steam.
Step away while it's venting, then when it stops (you'll hear it stop making noise), carefully open the lid. Again, don't let the steam get in your face, though it won't be as pressurized by now.
There will be quite a lot of liquid (sadly inevitable when dealing with the instant pot). Remove your four big roast chunks and place them on a cutting board, then use forks to shred. Return to juices and stir around to allow juices to coat the meat. Then spoon the mixture into a serving dish or plastic container (if you're saving to reheat later). As you're spooning, pick up the meat, tomatoes, and peppers but leave about half of the liquid behind. You can use this leftover liquid in a soup or simply discard.
And that's it! Serve on tacos, over rice, or in a taco bowl.
Instructions for Slow Cooker or Roaster: Once you have all your ingredients in the slow cooker or roaster, add about 1/4 inch of water to the bottom. Place lid on and turn to medium heat on the slow cooker or to around 200 degrees F on the roaster. Slow cook/roast for around 6 hours until meat is tender (checking water level occasionally to maintain at 1/4 inch).
Remove, shred, and serve or transfer to container.