Season the ribs on all sides with salt and pepper. Place vegetable oil and the ribs in a pan, turning once, until browned, about 3 minutes per side. Transfer the ribs to a slow cooker.
Blend the onions, scallions, peppers, tomatoes, garlic, cumin, achiote, panela or brown sugar, and cilantro with the water until you get a smooth puree.
Pour in the tomatoes and onion sauce and beer over the ribs. Add the carrots and potatoes. Cover and cook until the meat is separating from the bones and the vegetables are tender, 5 to 6 hours on high or 7 to 8 hours on low.
Spoon the fat off the sauce. Transfer the ribs and sauce to a serving plate and serve immediately over white rice.