You do not need to slice your deli roast beef into strips before adding them to the caramelized onions if you are short on time. Simply layer the roast beef slices onto the slider buns in the pan, then top them evenly with the caramelized onions. I prefer the method in the recipe because I like to warm my roast beef slightly and evenly distribute all my onions and beef. Also, when you slice your roast beef into strips, it helps when separating and serving these French dip sliders once they are baked.