Toss the chicken chunks with the turmeric and salt. Let them sit while you prep everything else, about 10 minutes. The turmeric seasons the meat and gives it a little color before it ever hits the sauce.
Heat the oil in a wide skillet or wok over medium-high. Sear the chicken in a single layer, undisturbed, for about 2 minutes per side until lightly browned. It does not need to cook through here. Lift it out and set aside.
Lower the heat to medium. In the same pan add the sliced onion, garlic, ginger, and bruised lemongrass. Cook 3 to 4 minutes until soft and fragrant.
Add the chili paste and fry it for 2 full minutes, stirring, until the oil turns red and the raw smell is gone. This step is what makes the sauce taste cooked instead of sharp, so do not rush it.
Stir in the tomato puree, ketchup, sugar, and water. Simmer 3 minutes until it thickens into a glossy sauce. Taste and adjust salt and sugar.
Return the chicken along with the onion petals and peas. Simmer 5 to 6 minutes, spooning sauce over the chicken, until the chicken is just cooked through and the sauce clings. Top with fried shallots and cilantro and serve.