Dice half the onion (you’ll need 1 cup diced) and thinly slice 1/4 of the onion ( 1/4- 1/2 cup).
Place the sliced onions in a bowl, cover with 1/2 cup hot boiling water and 2 tablespoons vinegar, place in the fridge.
Cook the meat: Heat oil in a large skillet over medium heat. Add 1 cup diced red onion, saute 3-4 minutes, add garlic, lemongrass and chili flakes. Saute until fragrant, 2-3 minutes. Add the ground meat, breaking it apart with a metal spatula. Season the meat with the salt and pepper and continue cooking until all the liquid releases and evaporates about 10 minutes. Drain the fat from the meat, using a fine-mesh strainer.
While the ground meat is cooking, prep the remaining ingredients. Place the cucumber, radish, scallions, mint, basil, cilantro, fresh chili (optional) in a bowl. Add the drained meat and the strained pickled onions. Add the fish sauce, lime juice and sugar. Taste, adjust salt, lime and heat. I’ll usually add more chili flakes. or finely diced fresh chilies.
Serve with lettuce wraps and/or cooked rice. Serve warm or chilled. ( If chilling, make doubly sure to strain all the fat from the meat -it will harden in the fridge.)