To make filling: Combine cherries, 1/2 cup sugar and 2 tablespoons lemon juice in a saucepan. Heat over low heat, stirring occasionally, until the sugar dissolves. Simmer the cherries until very tender, 7 to 10 minutes. Whisk together 2 tablespoons Amaretto and cornstarch in a small bowl and add to the cherries. Increase heat to medium and cook, stirring constantly, until thick and glossy, about 1 minute. Remove from the heat and stir in lemon zest. Let cool to room temperature, about 1 hour.