Add the flour, salt and rosemary to a food processor. Pulse several times to chop the rosemary and combine the ingredients. Add the cold butter and continue pulsing on and off until the mixture is crumbly. Slowly add the water with the machine running just until the dough forms a ball. Chill for at least 2 hours or overnight.
Preheat the oven to 375 degrees F and line a baking sheet with parchment paper. On a lightly floured surface, roll out the dough into a circle about 9 inches in diameter. Use a knife or pizza cutter to even the edges and move the dough to the prepared baking sheet. Place in the refrigerator.
Line a plate with paper towels. Slice the plums in half and carefully remove the pits. Take one of the halves and cut it into thin slices and then place on the paper towels to drain excess liquid. Repeat with the remaining plums.
Remove the baking sheet from the refrigerator and spread the mascarpone cheese evenly onto the dough, leaving an inch border all the way around. Layer the plum slices in a decorative circular pattern, starting with the outer layer and finishing with an inner layer. Fold the excess dough on top of the plum in a circle, until you have a tightly formed tart.
Place in the oven and cook for 22-25 minutes, or until the dough is cooked and center is bubbly. Toward the end of the cooking time, place the apricot jam in a small ramekin and microwave for 30 seconds to thin it out.
Carefully remove from the oven. The center may appear to have a lot of liquid but will firm back up as the galette cools. Use a pastry brush to dab the apricot jam evenly over the top, taking care not to disturb the shape of the plums.
Use a pepper grinder to add a light sprinkling of fresh ground black pepper evenly over the plums.
Allow the tart to cool until it's lukewarm or at room temperature before serving.
Do not use overly ripened plums or they will be too water and will not hold their shape when sliced. Look for firm plums that give just slightly when pressed.