Rinse the chicken pieces and pat dry. Place them in a large bowl and season with scallions, garlic, ginger, thyme, 1 tablespoon curry powder, salt, black pepper, and allspice. Mix well and let marinate for at least 15 minutes.
Heat vegetable oil in a heavy-bottom pot over medium heat. Add the remaining 1 tablespoon curry powder and stir continuously for about 30 to 45 seconds until fragrant. Do not let it burn.
Add sliced onions to the pot and sauté until softened. Add the seasoned chicken and stir to coat it well with the curry base. Allow the chicken to brown lightly for 5 to 7 minutes.
Pour in the water, add potatoes and carrots, then cover and reduce heat to low. Simmer for 30 to 35 minutes, stirring occasionally, until the chicken is tender and the sauce thickens naturally.
