Combine the beef with soy sauce, beer, mustard, cumin, pepper, and salt. Rest for 20 minutes.
Parboil the potato wedges for 5 minutes, drain thoroughly, and cool. Fry them in batches in half the oil until golden and tender; drain and keep warm.
Sear:
Brown the sausage in a large skillet and set aside. Add the remaining oil and sear the drained beef in two batches so the strips brown without steaming.
Cook the onions in the same skillet for 4 minutes. Add the tomato wedges and half the locoto for 2 minutes, then return the beef and sausage with 1/4 cup of the marinade. Cook until glossy but not saucy.
Assemble:
Spread the potatoes over a large platter. Spoon the beef, sausage, onions, tomatoes, and pan juices over them.
Top with egg quarters and remaining locoto. Serve immediately with lime wedges.
Notes
Locoto peppers can be very hot; seed them or substitute a milder red chile.
Use a broad platter so the potatoes stay crisp beneath the hot topping.