PREP: Preheat the oven to 400 degrees F. Very generously spray a glass, ceramic, or glass (not metal) 9x13-inch pan. Cut butter into 6 thin slices and set aside.
BAKE: Add all of the ingredients except the butter into the pan. Season to taste with salt & pepper (I add 1-1/4 tsp fine sea salt & 1/2 tsp pepper). If desired, add a few full sprigs of cilantro on top. Stir with a wooden spoon until ingredients are well incorporated (make sure the tomato paste is integrated). Add cut pieces of butter evenly on top of the dish. Cover tightly with foil. Bake for 40 minutes. Then, remove the foil (save foil -- just set aside for now) and bake for 15 more minutes.
REST: Remove from the oven. Don't stir the rice even though it looks like it didn't work out! Top with the foil again and let stand at room temperature for 10 minutes. This is very important to avoid mushy rice! Remove the cilantro sprigs and discard.
ENJOY: After 10 minutes, add some freshly chopped cilantro and fresh lime juice. (If adding, add to taste; we add about 1/3 cup cilantro and 1-2 tablespoons lime juice.) Gently mix everything together and lightly fluff rice with a fork. Taste for seasonings, adding extra salt/pepper if needed.