Chicken with caramelized onions, orzo & melted cheese

200 g orzo 4 boneless, skinless chicken thighs 4 large onions, thinly sliced 3 tbsp olive oil 2 tbsp balsamic vinegar 1 tbsp butter 100 g grated graviera cheese 700 ml stock (1 vegetable cube diluted in water) 1/2 glass white wine 1 tsp thyme or oregano Salt pepper