Rinse the pinto beans thoroughly under cool water. Remove any debris, stones, damaged beans, or beans that float. Set aside.

Place the rinsed beans in a large pot and cover with at least 3 inches of water (approximately 3 quarts). Add half of the onion, the salt, and the oregano. Bring to a boil, then reduce to a low simmer. Cover and cook for approximately 2 1/2 hours, or until the beans are tender and the skins begin to split.

If the beans remain firm, add 1/8 teaspoon baking soda and continue cooking for up to 30 minutes longer.

Place the beans, 8 cups (2 quarts) of water, half an onion, the salt, and oregano into a 4-quart or larger pressure cooker. Cook on high pressure for 30–35 minutes. Allow a natural release for 10 minutes before opening the valve.

If the beans are not fully tender, add 1/8 teaspoon baking soda and cook under pressure for an additional 10 minutes.

Note: Follow all safety guidelines for your specific pressure cooker model.

Drain the beans, reserving at least 1 cup of the cooking liquid. Remove the onion pieces and oregano stem, if used.

Finely chop the remaining half onion. Heat the bacon fat or olive oil in a wide, sturdy frying pan over medium-high heat. Add the chopped onion and cook until it becomes translucent.

Add the cooked beans to the pan along with 1/4 cup of the reserved cooking liquid. Using a potato masher, mash the beans to form a rough purée while they cook. Add additional bean liquid or water as needed to achieve your preferred consistency.

Season with additional salt if necessary. Add chipotle powder if desired. Continue cooking until the beans are heated through and have reached your desired texture.

Transfer to a serving dish. Top with crumbled cotija or grated cheddar cheese, if using.