1. Start by heating a large skillet over medium heat. Add the butter and let it melt, swirling it around to coat the bottom of the pan.
2. Add the sliced onions to the skillet and sauté for about 5 minutes, or until they start to soften and turn golden brown.
3. Stir in the minced garlic and sliced mushrooms, seasoning with salt and pepper. Cook for another 5-7 minutes, until the mushrooms are tender and cooked down.
4. While the mushrooms are cooking, spread a thin layer of Dijon mustard on one side of each slice of bread. This little step adds a great depth of flavor.
5. Once the mushrooms are ready, layer the mushroom mixture on top of two slices of bread, followed by two slices of Swiss cheese.
6. Top with the remaining slices of bread, mustard side down, creating a sandwich. Wipe out the skillet, then return it to the heat.
7. Toast the sandwiches in the skillet for about 3-4 minutes on each side, or until the bread is golden and the cheese is melty. You can cover the skillet with a lid to help the cheese melt faster!
8. Once done, remove from the skillet, slice in half, and serve warm. Don’t forget to share a bite with the kids — they’ll love it!

As a little tip from my kitchen to yours, feel free to experiment with different cheeses or add some fresh herbs like thyme or parsley for an extra pop of flavor!