For the Chicken: In a large pot, place chicken, onion, garlic, 1 ½ teaspoons salt, sazón with coriander and annatto, garlic powder, cumin, oregano, pepper, bay leaf, and 2 quarts cold water. Bring to a boil over medium heat, then reduce heat to medium-low and simmer until chicken is cooked through, about 10 minutes.

Remove from heat. Strain broth and set aside 4 cups broth. Using tongs, transfer chicken, onion, and garlic to a large food processor, and add remaining ½ teaspoon salt. Let sit for 10 minutes to let chicken cool slightly. Pulse until chicken is shredded very fine, about 1 minute. Wipe pot clean.

For the Rice: Add 2 tablespoons olive oil to now-empty pot and heat over medium-low heat until shimmering. Add salt, sazón, garlic powder, and turmeric, and cook until aromatic, about 30 seconds.

Stir in rice and cook until rice is coated with seasoning, about 30 seconds. Add reserved 4 cups broth and tomato sauce, and bring to a boil. Reduce heat to low, cover, and simmer until rice is fully cooked, about 16 minutes. Let rice sit for 10 minutes covered, then fluff rice with fork. Stir in shredded chicken and set aside.

Assemble the Arroz Imperial: Preheat the oven to 375°F. Layer half of shredded chicken and rice mixture in a 9x13 baking dish. Spread ¼ cup of mayonnaise evenly over top, then sprinkle with 1 cup cheese. Repeat with rice mixture, mayonnaise, and cheese, and sprinkle the top layer with a pinch of salt. Arrange pimiento peppers slices in the center of the casserole. Bake until cheese is melted and hot, about 10 minutes. Broil until cheese is golden brown, 2 to 4 minutes. Let sit for 5 minutes before eating.