Drain and prep: Open the cans of sardines and drain excess liquid. Remove scales if needed. If packed in oil, reserve about 3 tablespoons of the oil for cooking.
Sauté aromatics: In a heavy-bottomed pot, heat reserved oil or 3 tablespoons vegetable oil over high heat. Add onions and hot pepper, and cook until the edges start to brown.
Cook curry base: Stir in curry powder, green seasoning, garlic, and 1/2 cup water. Cook for about 5 minutes until the mixture becomes grainy and fragrant.
Add vegetables: Add the sliced potatoes and chopped tomatoes. Stir to coat everything in the curry. Reduce heat to medium-low, cover, and cook for 10 minutes. Check occasionally to prevent sticking. Add 1/2 cup water if necessary.
Add sardines and herbs: Once potatoes are fully cooked and tender, gently stir in the sardines along with scallions and culantro or cilantro. Cook for 1–2 minutes, breaking up large chunks gently.
Simmer: Add 1–2 cups hot water and let simmer for about 5 more minutes until the curry thickens but is not completely dry.
Final seasoning: Taste and adjust with salt and black pepper as needed. Serve warm.