Place flour, salt and cut up butter into a food processor and pulse until mixture is crumbly and looks like wet sand. Slowly add water into running machine until a dough ball forms. Shape into a disk, cover in plastic wrap and chill for at least 1 hour or freeze for 20 minutes.
Thinly slice the peaches and plums and place slices on paper towel to absorb excess liquid.
Preheat your oven to 375 degrees F. Roll out chilled dough and transfer to a lined baking sheet. Spread the dough with mascarpone cheese while leaving a 1-inch border. Arrange fruit slices in circular pattern over the mascarpone layer. Fold over the edges.
Bake for 20-25 minutes until fruit is bubbly and crust lightly browned. In the last minutes of baking microwave the apricot jam for 30 seconds. Carefully brush/dab jam over hot galette (fruit and crust). Cool slightly before serving.