Put chicken, garlic, ginger and curry powder in a large bowl and toss well to coat. Marinate while you prepare everything else.
Put oil in a large, nonstick skillet/wok; turn heat to medium. Add onion and cook, stirring occasionally, until soft and translucent, about 5 minutes. Raise heat to medium-high and add chicken. Cook, stirring occasionally until the chicken loses its raw look and begins to brown, about 5 minutes. (Additional cooking is coming when its in the sauce.).
Stir in peanuts, sugar, nam pla and coconut milk. Cook, stirring occasionally, until the sauce is thickened and chicken is tender, 5 - 10 minutes. Serve immediately.
NOTE: You can also use 1/2 head of broccoli and 6 scallions (2-inch lengths) in this also. Just parboil them first for about 3 minutes or until barely tender before adding to pan along with chicken. Obviously, you'd drain the vegetables first. You'd also need to increase the coconut milk to 3/4 cup total.