Peel the onion and chop into 2-inch chunks. Add onion, chicken broth, and water to a pot and bring to a boil; simmer uncovered until nearly all liquid has evaporated, about until very soft. Let cool slightly.
Puree the cooled onion in a food processor until smooth and set aside.
Heat the vegetable oil in a Dutch oven or heavy pan over medium-low heat. Add the mustard, cumin, and coriander seeds and toast, stirring, until they begin to sizzle and become fragrant, about 2–3 minutes; do not burn.
Carefully add the onion puree (it may splatter) and cook, stirring, for about 7 minutes over medium heat.
Add powdered ginger, ground turmeric, minced garlic, and yellow curry powder; stir and cook 1–2 minutes until spices are fragrant.
Add the chicken pieces and sauté about 2 minutes until they begin to seal.
Stir in the tomato paste, coconut milk, and soy sauce until combined. Add the chopped bell pepper and drained pineapple chunks.
Bring the curry to a gentle boil, then cover loosely and simmer for 10 minutes, or until chicken is cooked through and peppers are tender.
Taste and season with salt if needed, then finish with lime juice to taste. Serve garnished with chopped peanuts, green onion, and cilantro over rice if desired.