Heat the oil in a large cast-iron Dutch oven or a wok over medium-high heat. Add the onions and cook, constantly stirring, until they turn golden brown.
Add the carrots and continue cooking for another 3 minutes.
Finally, add the chopped tomatoes and cook for another 3 minutes.
Add the stuffed bell peppers, potatoes, carrots, garlic, one tablespoon of salt, and crumbled beef bouillon cubes.
Next, add enough water to cover the peppers and the vegetables. If some parts stick out, that's not a problem, as they will cook from the hot steam inside the oven.
Bring the soup to a boil, lower the heat to medium-low, and cook, covered, for 1 hour and 20 minutes. About halfway through cooking, check the soup for salt and adjust if needed. After 1 hour and 20 minutes of cooking, check the pepper filling for doneness; the rice should be perfectly done. If not, cook for a little longer until perfectly done.
Serve hot, garnished with chopped fresh greens (cilantro, parsley, basil, etc.) and a dollop of sour cream, if you like it. Be careful not to break the peppers when transferring to serving plates. A large serving spoon works best for that.