Place a large, deep skillet over medium heat and coat with the oil. Saute the onion and green peppers for 3 minutes to soften. Split the piece of lemongrass down the middle and whack it with the flat side of a knife to open the flavor. Add the lemongrass, ginger, curry paste and lime leaves to the skillet and stir for 2 minutes. Pour in the coconut milk and chicken broth. Lay the chicken pieces in the mixture to poach; add a pinch of salt. Stir together and simmer over low heat for 10 to 15 minutes. Squeeze in the lime juice and shower with basil and cilantro; serve in dinner bowls with lime wedges.
Heat the coriander, cumin seeds, and peppercorns in a small dry skillet for 2 minutes until fragrant. Put the seeds in a clean coffee grinder or spice mill and buzz the spices to a powder.
Put the spice blend and remaining ingredients in a food processor, and pulse to combine. Pour in the water to help grind everything down into a paste.