Preheat oven to 375F.  Line a baking pan with parchment paper. Butterfly each chicken breast. Place your knife along the side of the breast and use a sawing motion to carefully slice through the side, like opening a bagel. Be careful not to slice completely through the other side. Open the breast and lay flat.

Sandwich each chicken breast between two pieces of plastic wrap. Using a meat tenderizing mallet, pound each breast until it is flat and about ¼ inch thick. Sprinkle each side lightly with salt and pepper.

Heat a medium sized skillet over medium heat. Add butter. When butter stops foaming, add onions to the skillet. Cook onions over medium-low heat until they caramelize, stirring frequently.

Add the white wine vinegar and stir, scraping up the browned bits. When the vinegar becomes syrupy, remove your pan from the heat. Add about 1/3 of the sage and stir to combine. Season to taste with salt and pepper.

Lay out three pieces of twine and place the flattened chicken breast on top. Keeping the filling away from the edges, spread ½ of the onion sage mixture on each chicken breast. Sprinkle some grated cheese over each breast, reserving a smaller portion for later.

Roll each breast and secure tightly with twine or toothpicks.

Place in your baking pan and make sure they do not touch. Sprinkle reserved cheese and sage over the roulades.

Bake in preheated oven for about 35 minutes or until chicken is cooked through. If the top is not sufficiently brown, place chicken under the broiler for a few minutes at the end of cooking. Watch carefully as the cheese will brown quickly.

Remove from oven and allow to cool about 5 minutes before slicing.  Remove twine or toothpicks from the roulades. Carefully slice in a crosswise fashion and serve.