Heat oven to 400ºF.

In a large sauté pan over high heat, heat 1 tablespoon of olive oil. Add the onions and a pinch of salt, and immediately turn the heat to medium-low. Stir every so often, and cook until the onions soften and begin browning at the edges, about 5 minutes.

Meanwhile, place the chicken in a large bowl and season all over with kosher salt. Use 1 teaspoon per pound.

Add the curry powder, turmeric, and Thai red curry paste to the onions, and stir until onions are coated in the spices, about a minute. Add the tomatoes, and stir until they just begin to soften, about a minute. Add the can of coconut milk. Fill up the empty can of coconut milk with water and add to the pan. Add the fish sauce and brown sugar and bring to a simmer.

Add the chicken pieces and turn to coat. Transfer pan to the oven uncovered and cook for 1 hour or until the liquid has reduced considerably, the chicken skin is brown, and the meat is falling off the bone. If the liquid has reduced too much, add more by the quarter cup.

Add cilantro and stir. Add spinach, if using, and stir. Spoon rice into bowls. Top with chicken and sauce, and serve immediately.