Heat skillet with the tablespoon oil over medium heat. Add onion; cook and stir until onion is translucent, about 5 minutes. Break frozen steak pieces into skillet; cook and stir until steak is no longer pink, about 10 minutes. Remove from heat and set aside.
Arrange egg roll wrappers on a flat surface; place a half slice of cheese on each one. Layer an equal amount of steak mixture on top of each cheese slice. Roll up egg rolls according to package directions, and seal edges, wetting with water if necessary.
Heat 1 inch of oil in a large skillet, or preheat a deep-fryer to 375 degrees F (190 degrees C).
Fry the rolls in the hot oil until golden brown on all sides, about 5 minutes. Remove; drain on paper towels. Serve warm with ketchup for dipping.
