Season the beef with salt and pepper.
In a 10-inch cast-iron skillet over medium-high heat, heat the vegetable oil. Sear the beef in four batches until browned, about 2 minutes per batch. Aim for it to be just browned and coated with oil, not overcooked. Use a slotted spoon to transfer the beef to another container.
In the same skillet over medium heat, sauté the yellow onion and cumin seeds until the onion is translucent, approximately 3 minutes. Add the tomatoes, tomatillos, garlic, pasilla, dried chiles, and bay leaves. Cook for 30 minutes, stirring occasionally, until the tomatillos are tender and the chiles are soft. Reduce the heat to medium-low and incorporate water to maintain a saucy consistency. Pour the mixture into a blender and process until the sauce is as smooth as possible.
Return the beef to the pan and cover it with the sauce. Serve the dish family-style with tortillas and red onions, allowing everyone to create their own tacos.