Freeze beef for 2 hours. This helps you get the thinnest slices possible.

Heat a cast iron skillet over medium-high heat and add three tablespoons of canola oil, then the onions.

Cook until onions wilt. Add salt and pepper. 

Remove onions from pan, and return skillet to burner. Turn heat to high.

Add in the last tablespoon of oil. Season beef with salt and pepper, then add to the skillet. Sear quickly, watching so the beef doesn't get too dry. 

Turn heat down to medium, and add onions to beef. 

Warm Cheez Whiz in microwave-safe bowl.

Toast sub rolls (in toaster or on warm griddle). Spread mayo on bottom half of roll. Add onions and beef, top with Cheez Whiz.