Start by heating the olive oil and butter in a skillet over medium-low heat. Once melted, add the thinly sliced onions. Stir them to coat. Sprinkle a little sea salt to help draw out the moisture. That’s the ticket! Now we wait. Stir occasionally over the next 20–25 minutes. Watch as the onions transform and gradually become golden brown and sweet. Add the thyme toward the end for a fragrant finish.

When your onions are perfectly caramelized, it’s time to add the sherry wine or broth. This step deglazes the pan, pulling off the flavorful bits stuck to the surface and intensifying the overall flavor. Let the liquid reduce for a few minutes until it’s nice and thick. Remove from heat, and set your precious onions aside.

Heat a separate skillet on medium heat. While it warms up, spread a thin layer of whole grain mustard on one side of each slice of your bread. Next, pile some of the delicious caramelized onions onto two slices, followed by a generous amount of Gruyère cheese. Top each with the other slices to create your sandwiches.

Put a bit of butter or olive oil in the pan. Once it’s sizzling, place your sandwiches in the skillet. Grill them for about 4–5 minutes per side until they're golden brown and the cheese is oozy and melty. Press down slightly for even grilling!

Once grilled to perfection, slice your sandwiches in half, making them easier to handle. Drizzle with balsamic glaze if you want to take it to the next level! Serve them piping hot alongside your chosen sides.