To make onions: Heat 2 tbsp butter and olive oil in a large skillet over medium-low heat. Add sliced onions and salt.

Cook onions slowly, stirring occasionally, for 30-45 minutes until deep golden brown and jammy. Stir in fresh thyme in the last minute. Remove from pan.

While onions cook, thinly slice the pear and apple, and grate the cheeses.

Butter one side of each bread slice. Place two slices, butter-side down, on a work surface.

Layer half the cheese, followed by fruit slices, caramelized onions, and the remaining cheese. Top with remaining bread, butter-side up.

Wipe out the skillet and return to medium-low heat. Place sandwiches in the skillet.

Cook for 4-5 minutes until the bottom is golden brown. Flip carefully.

Cover the pan loosely with a lid and cook for another 3-4 minutes until the second side is golden and cheese is fully melted.

Let rest for 1 minute before slicing in half. Serve immediately garnished with fresh thyme.