Guinness Irish cheddar whiz: sprinkle and coat your shredded cheddar cheese with the cornstarch and set aside.
In a small saucepan over medium heat add evaporated milk and Guinness and bring to a simmer.
Add shredded cheddar a small amount at a time until melted. Then add whiz, stirring to incorporate.
Season with garlic powder, mustard powder, salt, and white pepper to taste.
Store in a sealed container in your refrigerator for up to a week.
Sandwich cooking and assembly: saute diced onions in olive oil with a pinch of salt until caramelized (about 5 mins) set aside. Stack corned beef and cut into long strips.
On a griddle or cast iron pan, heat 1 teaspoon of cooking oil and add your beef and cook untouched for about a minute. Begin to chop your beef with a flat spatula or bench scraper, add onions and mix chop to combine.
Add a few tablespoons of your cheese sauce and chop together with the meat and onions.
Open your bread roll and place atop your meat mixture to steam for 30 seconds. Scoop up everything and tuck into bread.
Top with more Irish cheddar whiz and sauerkraut.
Wrap sandwich in parchment for a better effect.