First you need to caramelize the onion. Slice the onion into long thin even slices. It's important that they are even so that the onion caramelizes evenly. Then warm the olive oil in a heavy bottomed skillet or sauce pan over medium low heat. Add the onions. Stir frequently, until they reach a golden color. You can increase the heat to medium once the onions are soft - but only if you stir them every minute. This is more labor intensive, but it speeds up the process, which will take at least 30 minutes.

Once the onions are caramelized you are ready to assemble your sandwich. I started by slicing the rolls in half, adding a nice layer of spicy mustard on both sides of the roll, and then layering the spinach, roast beef, blue cheese, and caramelized onions on top. Eat right away so that the onions don't make the bread soggy.