I sometimes think I could live on Vietnamese food. I love the key flavors and I adore the balance of hot, sour, salty, and sweet that is such a dominant characteristic. This is incredibly easy, somewhere between a stir-fry and a sauté, with just enough sauce to coat the pieces of chicken.
Trim any fat from the thighs and cut the flesh into bite-sized pieces. Remove the tough outer layers of the lemongrass and trim the top and base. Mince the rest—the softer part of the lemongrass—as finely as you can. Put half of this into a bowl with the fish sauce, sugar, half of the chiles, all the garlic, and the chopped chicken. Mix together with your hands, cover with plastic wrap, and put into the fridge. Let marinate for at least 4 hours; overnight is even better. Bring it to room temperature before cooking.
Heat the oil in a wide-based saucepan or a wok set over medium heat. Add the chicken and cook on all sides, getting a really good color all over it. Add the reserved chiles, reserved lemongrass, and the onion and stir-fry until the onion starts to soften. Be careful not to burn the chiles or the lemongrass. Pour in the stock, reduce the heat, and cover. Cook the chicken for about 5 minutes.
Remove the lid, increase the heat, and continue to cook until the liquid is reduced. You want a mixture that is wetter than a stir-fry but drier than a braise. Add lime juice to taste, then check for seasoning and sweet-sour balance, adjusting with sugar and lime. Scatter with the amaranth or cilantro leaves and sesame seeds, if using, and serve with brown rice and stir-fried greens.