Sift together flour, sugar, and salt. Rub in butter with fingers, until the mixture resembles a fine meal. Stir in the yogurt, mix until dough holds together. Wrap in plastic wrap and chill for an hour.
Mix plums and the rest of the filling ingredients.
Let dough stand at room temperature for five minutes. Roll out pastry to about a 13 inch circle. Place on a parchment-lined cookie sheet.
Arrange plums on the dough, leaving a couple of inches around the edge free of plums. Fold that edge inward, leaving a large center circle of the plum mixture exposed.
Mix beaten egg and cream together for an egg wash, brush onto dough, sprinkle with sugar. Bake at 375F for 49 to 50 minutes.
Let galette cool a bit, slide it onto a serving plate, and serve plain, with whipped cream, or with vanilla ice cream.