Put the prunes in a bowl, squeeze the lemon juice over the prunes, pour the Muscat over them and sprinkle with 1 tablespoon of sugar. Toss to coat and let sit for 30 minutes.
In the meantime, preheat the oven to 325 degrees F grease the pie dish with butter and dust with sugar. In a mixing bowl, sift together the remaining 1/4 cup sugar, flour, and salt. Add the eggs, milk, and vanilla to the dry ingredients and beat until smooth.
Using a slotted spoon, remove the prunes from the Muscat and arrange them in the bottom of the prepared pan. Pour the batter over the fruit and sprinkle the top with the slivered almonds.
Transfer the clafoutis to the oven and bake for 30 to 40 minutes, until risen and lightly firm.