Set an oven rack near the broiler and heat the broiler to high. On a foil-lined sheet pan, spread the tomatoes, jalapenos, serranos, poblano, onion, and garlic. Broil 6 to 7 minutes, turn everything, then broil another 6 to 7 minutes, until blistered and charred in spots. Pull the garlic early if it darkens too fast.
Transfer the charred peppers to a bowl and cover for 10 minutes to steam. Peel off the loose skin, then remove the stems and seeds. Leave some charred skin on for extra smoky flavor if you like, and leave the seeds and innards in the jalapenos and serranos for more heat.
To a blender, add the charred tomatoes, jalapenos, serranos, onion, peeled poblano, garlic (squeezed from the skins), cumin, oregano, beef broth, and salt to taste. Blend until smooth. Taste and adjust the salt, then set aside.
Heat the oil in a large skillet or cast-iron pan over medium-high heat. Add the potatoes with a pinch of salt and cook, tossing now and then, until golden and just tender, about 10 minutes. Remove to a plate and set aside.
Season the sliced beef with salt. In the same skillet, sear the beef in a single layer until browned and nearly cooked through, about 5 minutes. Work in batches so it sears instead of steams.
Pour the ranchero sauce over the beef and bring to a simmer. Cook until the sauce thickens slightly and the flavors come together, 7 to 8 minutes. Add a splash of broth or water if it tightens up too much.
Fold the potatoes back in and warm through, 2 to 3 minutes. Taste for salt, top with fresh cilantro, and serve with warm tortillas, rice, and beans.