01 -
In a skillet over medium heat, warm olive oil and sauté onions with a pinch of salt until caramelized and golden, about 15–20 minutes. Set aside.
02 -
Butter one side of each slice of sourdough bread. On the opposite side, spread a thin layer of Dijon mustard.
03 -
Place two slices of bread, butter side down, in a skillet. Layer with cheddar, Gruyère, caramelized onions, a sprinkle of black pepper, and thyme if using. Top with the remaining bread slices, mustard side in, butter side out.
04 -
Cook over medium-low heat, pressing gently with a spatula, until the bread is golden brown and the cheese is melted, flipping once, about 3–4 minutes per side.
05 -
Serve the sandwiches hot and enjoy the gooey, savory perfection.