Heat cooking oil in a large pot or Dutch oven over medium heat. Add the diced green bell pepper and cook, stirring occasionally, until softened, about 5-7 minutes. This builds a foundational flavor base for the stew.

Add the drained and rinsed red kidney beans and cannellini beans to the pot. Stir well to combine with the peppers.

Add the undrained diced tomatoes, water or vegetable broth, salt, and black pepper. Bring the mixture to a gentle simmer.

Here's the secret to that incredibly rich broth! Once simmering, use the back of a sturdy spoon or a potato masher to gently mash about 1/4 to 1/3 of the beans against the side of the pot. This crucial step releases their natural starch and creamy texture directly into the liquid, thickening and enriching the broth without the need for any additional ingredients, giving you that deeply satisfying consistency.

Reduce heat to low, cover, and let the stew simmer for at least 15-20 minutes, or up to 30 minutes, allowing all the flavors to fully meld and the broth to deepen. Stir occasionally to prevent any sticking.

Taste and adjust seasoning if needed. Ladle the hearty bean stew into bowls and garnish generously with fresh chopped cilantro before serving warm.