01 -
Preheat oven to 200°C. Toss sliced mushrooms and onions with olive oil, salt, and pepper in a mixing bowl. Spread evenly on a baking sheet and roast for 20-25 minutes, stirring halfway through, until tender and slightly caramelized. Remove from oven and set aside.
02 -
Heat a skillet or griddle over medium heat. Butter one side of each slice of bread. Place half the slices, buttered side down, on the skillet. Evenly distribute shredded Gouda cheese over the bread slices. Add a layer of roasted mushrooms and onions, then top with remaining slices of bread, buttered side up.
03 -
Cook sandwiches until bread is golden brown and cheese is melted, about 3-4 minutes per side. Press down gently with a spatula to ensure even melting.
04 -
Slice sandwiches in half and serve warm. Garnish with fresh thyme leaves if desired.