Begin by dicing the chicken breasts into bite-sized pieces. In a mixing bowl, coat the chicken with olive oil, then season with salt and black pepper.

In a large skillet, heat olive oil over medium heat. Add the chicken pieces in batches, cooking for about 5-7 minutes until golden brown, then set aside.

In the same skillet, sauté diced yellow onion for around 3-4 minutes until soft and translucent.

Stir in diced tomatoes, tomato paste, pumpkin puree, and coconut milk. Add lemon juice and mix well. Simmer for 2-3 minutes.

Add brown sugar, curry powder, mild chili powder, cumin, mustard seed, coriander seed, cinnamon, salt, and pepper to the sauce and stir.

Return the sautéed chicken to the skillet, cover, reduce heat to low, and let it simmer for 10-15 minutes.

Serve the curry over rice or your preferred grain, topping with fresh cilantro or coconut milk.