Place the pork roast in a crock pot. Drizzle 2 tablespoons of the olive oil over the roast and sprinkle it with salt and pepper. Cover with the lid and cook it on high for 6 hours (or low for 12 hours).
After the roast has been in the crock pot for 4 hours (8 hours on low), add the onions and carrots to the crock pot, placing around the roast (Alternatively, you can pan fry the onions and carrots with the remaining 2 tablespoons of olive oil and saute them on medium/high heat until they begin to caramelize. Add them to the crock pot after this step).
Cook the roast, carrots, and onions for 2 more hours (4 hours on low), or until the pork is pulls apart easily and carrots are soft.
Place the roast, carrots, and onions on a serving platter and drizzle with chimichurri sauce. Serve with extra sauce.