Sprinkle 2 teaspoons of salt on both sides of the chicken thighs and let them sit while you prepare the onions.
Heat 2 tablespoons of olive oil in a large sauté pan over high heat.
When the oil is hot but not smoking, add sliced onions and cook, stirring frequently, until lightly browned about 6 minutes.
Lower the heat to medium and stir in ground cumin and cinnamon, cooking for one minute.
Add minced garlic and cook for 30 seconds more.
Transfer the mixture to the slow cooker, leaving any remaining oil in the pan.
With the pan still on medium heat, add the chicken thighs skin-side down and cook until the skin is deeply browned, about 10 to 12 minutes.
Flip the chicken and brown the other side for 4 to 5 minutes.
While the chicken browns, add raw long-grain rice to the slow cooker and stir to combine with the onions.
Mix in diced tomatoes, peas (if using), chicken stock, black pepper, bay leaves, and 1 ½ teaspoons of salt.
Once the chicken is done browning, place it skin-side up on top of the rice mixture in the slow cooker.
Cover and cook on high for 2 to 3 hours, checking halfway through to see if the rice needs additional liquid, adding ½ to 1 cup of water if necessary.
When the rice is tender and the chicken is cooked through, fluff the rice with a fork.
Taste and add more salt if needed before serving.