Polenta, Goat Cheese, and Mushroom Tart

INGREDIENTS

• 1 cup cornmeal or instant polenta
• 1/4 pound soft mild goat cheese from a log or disk
• 1/2 teaspoon salt
• 4 cups water
• 2 large eggs, beaten lightly
• 1/2 pound mushrooms, sliced
• 1 teaspoon salt
• 1 stick (1/2 cup) unsalted butter, softened
• 1 1/2 cups all-purpose flour
• 1 tablespoon fresh small thyme sprigs
• 3 to 4 tablespoons ice water
• 1/2 cup sour cream
• 1 stick (1/2 cup) cold unsalted butter, cut into bits