Chop the onion and cut the chicken into 1-inch pieces. Toss the chicken with curry powder, salt, and pepper in a bowl.
Peel the garlic cloves and ginger.
Heat a little oil in a large skillet and cook the onion until softened, about 2 minutes.
Purée coconut milk, ginger, and garlic in a blender until very smooth.
Add the chicken and the sauce to the skillet. Cook together for 7 to 10 minutes until the chicken is tender and cooked through.
Make the optional mustard seed and cashew topping.
Fold spinach into the chicken mixture and cook until just wilted (about a minute).
Divide among bowls, top with cilantro leaves and cashew topping, and serve with naan or rice.
