Hot Sandwiches with “Rossiysky” Semi-Hard Cheese and Caramelized Onions

Instructions:
1.Caramelize the onions: Slice the onion thinly and sauté in butter with sugar and balsamic vinegar until golden and soft. Season with a pinch of salt and pepper.
2.Assemble the sandwich: Layer cheese, caramelized onion, a bit of mustard (if using), and more cheese between slices of bread.
3.Grill the sandwich: Toast in a skillet or sandwich press over medium heat until golden brown and crispy on both sides, and the cheese is melted.
Serve hot — best enjoyed straight from the pan!
Ingredients:
Bread — 4 slices
“Rossiysky” semi-hard cheese — 100 g
Onion — 1 medium
Butter — 1 tbsp
Sugar — 1 tsp
Balsamic vinegar — 1 tsp
Mustard — 1 tsp (optional)
Salt, pepper — to taste