Rinse the rice under cold water until the water runs clear. Drain and set aside.
In a large pot, heat the vegetable oil over medium heat.
Add the chopped onion and sauté until soft and translucent, about 5 minutes.
Add the minced garlic and chopped bell pepper, and sauté for another 2-3 minutes.
Stir in the diced tomatoes and tomato paste. Cook for about 5 minutes, until the tomatoes break down and the mixture thickens.
Add the ground thyme, ground cumin, paprika, ground cayenne pepper (if using), ground black pepper, and bay leaves. Stir well to combine.
Add the rinsed rice to the pot and stir to coat the rice with the tomato mixture.
Pour in the chicken or vegetable broth and bring to a boil.
Reduce the heat to low, cover, and simmer for about 20-25 minutes, or until the rice is cooked and the liquid is absorbed. Stir occasionally to prevent sticking.
Stir in the mixed vegetables and cooked chicken, beef, or shrimp (if using).
Cover and cook for an additional 5-10 minutes, until the vegetables are tender and the protein is heated through.
Remove the bay leaves and adjust seasoning with salt to taste.
Garnish with fresh chopped parsley or cilantro.
Enjoy your meal!