cabbage and mushroom galette

INGREDIENTS

• 1 1/4 cups all-purpose flour
• 1/4 teaspoon salt
• 8 tablespoons (1 stick) unsalted butter, cut into
• pieces
• 1/4 cup sour cream
• 2 teaspoons fresh lemon juice
• 1/4 cup ice water
• 2 tablespoons butter
• 1 large onion, finely diced
• 4 ounces fresh shiitake mushrooms, stems discarded, caps thinly diced
• 1 teaspoon chopped thyme or 1/2 teaspoon dried
• 1 teaspoon chopped tarragon or 1/2 teaspoon dried
• 1 tablespoon chopped dill or 1 teaspoon dried
• 6 cups thinly sliced cabbage, preferably Savoy, or 4 cups cabbage plus 2 cups other greens, such as beet, chard, or kale
• salt and freshly milled pepper
• 1/4 cup chopped parsley
• 1 hard-cooked egg, chopped
• 1/4 cup sour cream or yogurt
• 1 teaspoon tarragon vinegar
• 2 tablespoons melted butter
• 1/4 cup prepared horseradish
• 1 cup yogurt or sour cream