Make the green seasoning by blending onion, garlic, ginger, cilantro, culantro, green onions, and thyme with 1-2 tablespoons of water until coarse. Set 3/4 aside for the curry base and reserve 1/4 for finishing.
Pat chicken dry, remove skin, then toss with 1 tablespoon green seasoning, 1 tablespoon curry powder, salt, and pepper. Marinate for 15-20 minutes at room temperature.
Heat oil in a large pot over medium-high heat. Add remaining 2 tablespoons curry powder and cumin, stirring constantly for 30-60 seconds until fragrant and darkened. Do not burn.
Add the 3/4 portion of green seasoning to the bloomed curry and cook for 2-3 minutes, stirring frequently, until the paste darkens and becomes fragrant.
Add marinated chicken and cook for 8-10 minutes, stirring occasionally, until chicken is opaque and coated in the thick curry paste.
Add 2 cups water, potato chunks, and the whole scotch bonnet. Bring to a gentle boil, then reduce heat to medium-low, cover, and simmer for 25-30 minutes until chicken is tender and potatoes are fork-tender.
Remove scotch bonnet, stir in reserved fresh green seasoning, adjust salt to taste, and let rest 5 minutes before serving.