Grind onions, ginger, garlic, chillies and lemon grass in a small food processor until fine. Or pound in a pestle annd mortar. Add the chopped tumeric leaf or any herb that you are using to the processed paste.
Use about 2-3 tablespoons of this raw paste to marinate the chicken, add a couple of splashes of fish sauce and maybe a tablespoon of oil to keep the chicken moist. Rub the marinade over the chicken evenly. There is no need for salt as the fish sauce is salty. Place the marinated chicken breasts in a plastic bag and marinate overnight or for a couple of hours.
Meanwhile heat 2-3 tablespoons of oil in a small pot and saute the raw paste for about a minute, stirring so that it won't stick too much. Add coconut cream and tamarind juice. Add 1-2 tablespoons of the fish sauce and sugar. Stir and bring it to a simmer and simmer until the sauce reduces to a thickish paste.
Add salt to taste. Once done keep aside. Preheat oven to 190 C. Prepare chicken breast smearing the cooked paste over the top of the breasts evenly. Heat a thick bottomed pan and then pour in about 2 tablespoons of oil into it. When hot sear the chicken skin side down first. When golden turn over to sear the other side. This takes probably about a minute on each side.
Then place the seared chicken breasts on the rack of a roasting pan. Baste the chicken with more cooked paste on top, this time a little generously and roast in the oven for about 15-17 minutes. For the last 2 or 3 minutes turn on the broiler and broil until the top of the chicken gets slightly charred on the top.