Combine the curry paste and 2 tablespoons coconut milk in a large saucepan over medium heat. Keep stirring and scraping the mixture until it sticks to the pan and starts to brown, 5 minutes. Add the fish sauce and stir for a few seconds, then add the potatoes, peanuts, bay leaf, lemongrass, onion, the rest of the coconut milk, 1 cup water and 1/2 teaspoon salt. Simmer gently, stirring occasionally, until the potatoes are tender, 15 to 20 minutes. Meanwhile, toss together the sliced chicken with 1/2 teaspoon salt in a bowl and let sit at room temperature. Add the chicken to the sauce and simmer gently until just cooked through, 4 to 5 minutes. Remove the bay leaf and lemongrass. Serve with the rice.