In a large mixing bowl, combine the chicken cubes with the sea salt, curry powder, garam masala, yogurt, and lemon juice. Ensure each piece is well-coated. Cover and marinate in the refrigerator for at least 30 minutes or up to a few hours.
Heat sunflower oil in a skillet or Dutch oven over medium heat. Add chopped red onion and sauté until translucent and slightly browned, about 5-7 minutes. Add minced ginger and garlic, stirring for another minute until fragrant.
Add marinated chicken and leftover marinade to the skillet. Cook for 8-10 minutes, stirring occasionally, until chicken is browned and cooked through.
Stir in dry fenugreek leaves, curry powder, and garam masala. Add chopped canned tomatoes and water, mix well. Bring to a simmer and cook for about 15 minutes to meld flavors.
Reduce heat and stir in heavy cream. Simmer for another 5 minutes until sauce is creamy and rich.
Serve hot, garnished with fresh cilantro if desired. Pair with basmati rice or warm naan for a complete meal.