Set the Instant Pot to Saute on medium heat. Add the oil and onion and cook, stirring occasionally, until the onion softens and turns translucent at the edges, about 4 minutes. Add the garlic and cook until fragrant, about 30 seconds. Hit Cancel to turn off the heat.

Add the pinto beans, salt, cumin, oregano, and water. Stir to combine. Secure the lid and set the valve to Sealing. Pressure cook on High for 35 minutes.

When the cooking time ends, let the pressure release naturally for 15 minutes, then quick-release any remaining pressure. The beans should be completely tender and the liquid should look starchy.

Remove about 1 cup of the cooking liquid and set aside. Use a potato masher to mash the beans directly in the pot, or use an immersion blender for a smoother texture. Add back some reserved liquid if needed to reach your desired consistency - the beans will thicken as they cool.

Heat the bacon fat or oil in a large skillet over medium-high heat. Add the mashed beans in batches and fry, stirring and scraping the bottom, until the beans develop some caramelized spots and taste richer, about 5 minutes. This step transforms the flavor - do not skip.

Taste and adjust salt as needed. Transfer to a serving bowl and top with crumbled cotija. Serve warm with tortillas, as a side, or spread on tostadas.