In a medium bowl, stir together the cornstarch, turmeric and ½ teaspoon each salt and white pepper. Add the chicken and, using your hands, toss until evenly coated.

Line a rimmed baking sheet with a double layer of paper towels and set it near the stove. In a 12-inch skillet over medium-high or a 12- to 14-inch wok over high, heat the oil until shimmering. If using a skillet, add about half of the chicken in an even layer; if using a wok, add about one-third. Don’t crowd the pan. Cook, occasionally turning each piece, until golden brown on all sides, 5 to 7 minutes. As they finish, transfer pieces to the prepared baking sheet and add more to the pan as space allows. When all of the chicken has been fried, pour off and discard all but 2 tablespoons of the oil (it’s fine if some browned bits remain in the oil).

Return the pan to medium-high and heat the oil until shimmering. Add the lemon grass and two-thirds of the shallots; cook, stirring often, until the shallots are lightly browned, about 2 minutes. Add the garlic and ginger; cook, stirring, until fragrant, about 1 minute. Stir in the bell pepper and jalapeños, followed by the kecap manis and 3 tablespoons water. Increase to high and cook, stirring often, until large bubbles form all over and the sauce is syrupy, about 3 minutes.

Add the chicken and tomato; cook, stirring, until the sauce clings to the chicken, 2 to 3 minutes. Remove from the heat and add the remaining shallots and ¼ teaspoon white pepper. Taste and season with salt and white pepper, then remove and discard the lemon grass. Serve with lime wedges.