In a small bowl, combine the onions, hot water, and vinegar. Stir then cover and refrigerate.
In a small skillet, heat the oil over medium heat and add in the onions. Saute for 2 to 3 minutes.
Next, add in the garlic, lemongrass, and chili flakes. Cook for 2 to 3 minutes, stirring regularly, until fragrant.
Add the ground chicken and stir to mix well.
Season with salt, black pepper, and brown sugar, then continue cooking for 8 to 10 minutes until the chicken is cooked through.
While the chicken is cooking, stir together the cucumber, scallions, red chili, cilantro, mint, basil, fish sauce, lime juice, and brown sugar in a large bowl.
Once the chicken is done cooking, stir together with the salad and pickled onions.
Serve with lettuce leaves for wrapping or over rice.