Place a medium pan over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the onion, thyme, some salt and pepper to the pan, and put a lid on it. Cook the onions, stirring occasionally, until softened, about 10 minutes. Remove the lid and continue cooking the onions until deep golden brown, about 20 minutes (if the onions begin to stick to the pan too much, add a splash of water and scrape up any bits that are sticking). Transfer the onions to a bowl and cool.

To prepare the roll-ups, cut the crust off from both slices of bread. Roll the bread slices flat with a rolling pin or wine bottle. Spread about 2 tablespoons of caramelized onions onto each slice (you’ll have leftover caramelized onions – just keep them in the refrigerator for another yummy round of sandwiches). Divide the cheese and roast beef between the two sandwiches, making sure they’re spread out in an even layer and pushed out to the edges. With the long side of the bread facing you, roll it up and give it a good squeeze to make sure it’s tight. Cut both roll-ups on the bias into 3 slices each.

To make a simple sauce for the roll-ups, mix together the sour cream or yogurt and horseradish. Pack everything up and you’re set to go!