Heat the oil in the 2.3 qt aluminum pan (caldero) over medium heat, then add the onions, peppers, and garlic. Sauté until the vegetables are softened.
Add the beans with their liquid. Then add the squash, sazón, oregano, bay leaf, and bouillon. Stir to combine.
Add the tomato sauce and enough water to cover the beans (about 1 can). It's fine if the squash peeks out a little, but make sure the beans are fully covered.
Bring the beans to a simmer, then cover the pan and turn the burner down low. Let the beans simmer for 25 minutes.
Remove 1 ½ cups of beans and a few pieces of squash with some of the broth. Mash together in separate bowl.
Return the mashed beans to the pan. Taste the beans and season with salt and pepper if needed. Stir well, and cover the pan with a lid. Let the beans cook for another 10-15 minutes.
Once the beans have thickened, stir in the chopped cilantro and serve.