Finely chop onion, red bell pepper, celery, and white mushrooms.
In a large pot over medium-high heat, warm olive oil. Add onion, garlic, celery, and red bell pepper and stir, cook 2 to 3 minutes, until onion starts to soften. Season with a pinch of salt and pepper.
Add diced tomatoes and tomato past, stir and add cajun seasoning. Cook 2 more minutes.
Add kidney beans, soy sauce, dried thyme, white mushrooms, and rice. Stir and cook 2 minutes, then add 3 cups of water and a pinch of salt. Cover tightly and bring to a boil; reduce heat to medium-low and cook 20 minutes without lifting the lid.
Turn off heat and let stand 10 minutes before opening lid. Fluff rice with a fork and season with salt and pepper to taste. Garnish with cilantro (optional).