Add chicken, Jamaican hot curry powder, diced onion, chopped scallion, habanero pepper, thyme, garlic cloves, pimento, salt and pepper to a large bowl. Mix well to ensure each piece of chicken is thoroughly coated with seasoning. Cover and refrigerate overnight, or at least 6 hours.
Working in two batches (or more if needed), heat half of the vegetable oil in a heavy bottom pot such as a dutch oven over medium high heat until oil is shimmering and nearly smoking. Remove seasoning from chicken and set aside. Add chicken pieces to pot, being careful not to crowd the pot; the pieces of chicken should not touch. Brown chicken for 3-4 minutes per side, until medium brown but not cooked through.
Remove browned chicken from the pot and place on a plate. Pour off any excess oil from the pot so that roughly 2 tablespoons of oil remains in the pot. Return all the pieces of chicken to the pot and reduce the heat to medium low. Pour in hot water, making sure the water covers the chicken. Add more water if needed. Bring to a boil, then reduce heat to a simmer. Add diced potato if desired. Cover the pot fully and simmer for 25-20 minutes.
Taste a small piece of the chicken and sauce. Adjust the seasonings by adding additional salt and pepper as needed. Add diced tomato and seasonings. Return to a simmer. Cover and cook for an additional 10-15 minutes, or until the chicken is tender and fully cooked through.
Serve with rice. Enjoy!