Combine the walnuts, Chèvre, 1 tbsp. of the mint, and the sour cream in a small bowl. Stir well and set aside. In another bowl, lightly beat the eggs, salt pepper and remaining 1 tbsp. mint. Heat the oil in an 8-inch nonstick omelet pan over high heat. Add the egg mixture, and cook without stirring until the omelet starts to bubble around the edges, 10 seconds. Then stir, gathering the mixture toward the center of the pan. Cook until the bottom is set but the top is still slightly wet. Spread the Chèvre filling over half the omelet. Fold the other half over the filling, and cook until heated through, 30 seconds. Slide the omelet onto a plate and serve immediately.