In a pot boil the water with the bouillon cube, the shrimp shells, carrot (diced), and celery stalk. Boil until the carrot is fork-tender, then blend until you get a smooth soup.
In a big pot/ dutch oven, add the oil, onion (thinly diced), garlic cloves (minced), bell pepper (thinly diced), and tomato (diced). Let it cook over medium heat for a couple of minutes until the onions are translucent.
Toss the rice and the shrimp shell soup. Season with thyme, black pepper, red pepper flakes (optional), and salt if needed (taste before adding salt). As soon as it starts to boil, lower the temperature to low, cover with a lid, and let the rice cook for 15 minutes until it gets soggy. If the rice is absorbing too much liquid, add more water. The rice mustn’t dry.