Chop up the meat into large chunks and place in a big pot or dutch oven. Season with salt and pepper and mix.
In a pan toast/fry your chilies for a minute or two. If using dried chilies then place them in some water for 10 minutes after toasting to soften them up. Add to blender.
Grill the tomato halves for a minute or two, until the flesh is charred. Add to blender.
Fry the peppercorns, cumin, and cinnamon for just a minute on high heat. Add some water to the pan to dislodge anything stuck to the pan and then tip everything into the blender.
Add to the blender the garlic, cilantro, olive oil, vinegar, oregano, and the white onion. Blend everything up. If it is too thick add some water or beef broth to thin it down, but we don't want it TOO watery.
Pour the adobo (blended chili sauce) over the meat. Shake the pot to help the liquid seem through all the beef. Pour over just enough to cover the meat. Place the lid on and put into the fridge for at least 12 hours.
Preheat your oven to 160 Celsius and place the pot into the oven. Let braise for 5-6 hours, or until the meat is fork tender and easily shreds.
Strain the meat from the liquid (save the liquid!) and shred all the meat with a couple forks. Place in a serving bowl and ladle over some of the liquid you previously strained out. Mix everything together and enjoy.