For the beef: Heat 1/2-inch of the vegetable oil in a large skillet over medium-high heat. (When a crumb of panko sizzles after being dropped in, the oil is ready.)
Sprinkle the beef with salt and pepper. Place the flour, eggs, and panko in separate bowls. Dredge the beef strips first in the flour, shaking off any excess, then dip into the beaten eggs, and then the panko.
Fry the beef strips, turning once, until brown, about 1 minute per side. Transfer the beef to a paper towel-lined plate to drain. Sprinkle with salt.
For the sauce: In a small microwave-safe bowl, stir together the marmalade, vinegar, and mustard. Microwave on high for 45 seconds, or until bubbly. Serve with the beef strips.