Winter Greens and Apple Gratin

Name: Winter Greens and Apple Gratin
Ingredients: 1 1 big bunch of beet greens with stems (save the beets for another use - like beet cake!); 1 to 12 bunches of some other kind of winter greens, such as collards, kale, Swiss chard, or beet greens (you could even use some dandelion greens) stems removed; 1 1/2 tablespoons extra virgin olive oil; 1 onion, peeled and finely chopped; 1 large garlic clove, minced; 1 Fuji apple, cored and diced; 1/4 cup vermouth; 1 1/2 tablespoons Dijon mustard; 1 teaspoon Worcestershire sauce; 1 cup heavy cream; 1/3 cup grated gruyere cheese; 1/4 cup finely chopped walnuts; 1 pinch Kosher salt to taste; 1 pinch Freshly ground black pepper