Preheat the oven to 450˚F.
Set the roast on a large cutting board or platter. Combine the onion, bell pepper, garlic, salt, and cayenne and black peppers in a small bowl and mix well. With a sharp boning knife, make several deep slits in the roast spaced several inches apart. Using your index finger, stuff the seasoning mixture into the slits, packing it in firmly. Season the outside of the roast generously with more salt and cayenne pepper. Rub the roast lightly with vegetable oil.

Place the roast in a heavy roasting pan and place it in the oven. When the bottom of the pan begins to sizzle, carefully add the water. Bake the roast until it browns evenly, 30 to 45 minutes. More water can be added if the pan becomes too dry. This will mix with the roast drippings and make a dark gravy that can be used now for basting the roast, then later to pour over steamed rice.
When the roast is well browned, reduce the heat to 350˚F, cover and cook until the juices run clear and the roast is tender, 3 to 4 hours. 

Remove from oven and cool for 10 minutes before carving.