Preheat your oven to 350 degrees. Pound your halved chicken breasts to 1/4″ thickness
Heat up the olive oil in a large frying pan. Add the mushrooms and the leeks and cook for 5 minutes, or until the leeks are tender and fragrant. Stir in the balsamic vinegar, then parsley and transfer to a plate to cool.
Once the mushrooms are cool enough to handle, spread the mixture onto the flattened chicken breasts. Starting with one edge, roll the chicken breasts up and fix them in place with a toothpick. Transfer the chicken to a baking sheet and bake until cooked through (about 20 minutes). Pull out the toothpicks and serve warm.