Pre-heat oven to 425°F.
Add oil to a skillet, over medium heat, followed by onions and sauté for 10 - 15 minutes, until they begin to caramelize; set aside in medium bowl.
If using deli style roast beef, you do not need to cook; just season with salt and pepper and slice it thinly; add to the onions.
If using raw rib-eye, season the steak, slice it very thinly before searing over high heat; cook meat to desired temperature and add to onions.
Place wonton wrappers on a work surface, use a round cookie cutter, or cup and a knife, to cut out wonton circles.
Make sure you brush or spray both sides of the wontons rounds with cooking oil; be generous; this will allow them to crisp up nicely.
Using your fingers, press wontons into muffin tins.
Place in oven and bake, on the bottom rack for 6 - 8 minutes or until lightly browned.
Remove wonton shells from oven and fill each cup halfway with meat mixture.
Add a small slice of cheese; fill the rest of the tin all the way to the top of the wonton with more meat mixture.
Finally add another square of cheese to the top.
Place back in the oven, again on the bottom rack, until cheese is melted.
Remove from oven and serve with ketchup or more cheese sauce on the side.