In a large, heavy-bottomed skillet, heat the vegetable oil and butter over medium heat until the butter is melted and shimmering.
Add the chopped onions and sauté, stirring occasionally, for 8-10 minutes until they are soft, sweet, and golden brown.
Add the minced garlic and cook for another minute until fragrant.
Pour in the cooked kidney beans and their reserved cooking liquid.
Bring the mixture to a gentle simmer and reduce the heat to medium-low.
Cook, stirring often, allowing the flavors to meld and the liquid to reduce, about 10-15 minutes.
Using a potato masher or the back of a sturdy spoon, begin mashing the beans directly in the skillet. For a fantastic texture, aim for about half-mashed, leaving some whole beans for contrast.
Continue to cook and stir as you mash, for another 10-15 minutes, until the beans have thickened into a rich, creamy paste.
Season generously with salt to taste.
Serve immediately as a side dish topped with a sprinkle of grated cheese, or use as a delicious filling for tacos and burritos.
While kidney beans are delicious, feel free to substitute with traditional pinto beans or black beans for a different flavor profile.
