Combine the rice, onion, salt, and boiling water in a small saucepan. Cover and cook over low heat until the liquid is absorbed and the rice is just tender, about 10 minutes. Remove from heat and let cool slightly.
Heat the oven to 400°F/200°C. Spread the chopped red onion evenly over the bottom of a wide Dutch oven or deep baking dish. Spoon the rice into small mounds, leaving space between them. Set one chicken thigh over each mound and gently fold it around the rice. Nestle the shallots between the thighs.
Stir the marinade ingredients until smooth. Brush the chicken generously, reserving about 2 tablespoons. Scatter the chili slices over the dish. Carefully pour the boiling water down the side of the pot, avoiding the chicken. Cover tightly and bake for 1 hour.
Uncover and continue roasting until the chicken is deeply browned and glossy, about 15 minutes. Brush with the reserved marinade, sprinkle with sesame seeds, and serve hot.