Step 1
Place a rack in middle of oven; preheat to 350°. Gently toss together 2 lb. mixed stone fruit (such as peaches, nectarines, plums, and/or apricots), sliced ½” thick, zest and juice of 1 medium lemon, ½ cup (100 g) granulated sugar, 2 Tbsp. cornstarch, 1 Tbsp. vanilla bean paste or vanilla extract, and ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a large bowl until fruit is evenly coated. Let sit at least 10 minutes and up to 30 minutes.
Step 2
Meanwhile, mix 1 large egg yolk, 8 oz. cream cheese, room temperature, ¼ cup (50 g) granulated sugar, 3 Tbsp. all-purpose flour, ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, remaining zest and juice of 1 medium lemon, and 2 tsp. vanilla bean paste or extract in a medium bowl with a rubber spatula until smooth.
Step 3
Mix ½ tsp. ground cinnamon and remaining ¾ cup (150 g) granulated sugar and remaining ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt in a small bowl to combine.
Step 4
Working quickly to prevent phyllo from drying out, lay 1 of 10 sheets fresh phyllo pastry or frozen, thawed, lengthwise down center of a parchment-lined baking sheet. Brush some of 6 Tbsp. unsalted butter, melted, over pastry to create a thin layer, sprinkle a thin layer of cinnamon sugar over. Place another sheet of phyllo on top, situating at an angle so sheets form an X, (ends of pastry will hang over edges of baking sheet). Brush with butter and sprinkle with cinnamon sugar. Repeat process with 5 more sheets of phyllo pastry, crisscrossing each sheet at a wider angle from the original one to create a circle. (You should have some butter and cinnamon sugar left over.)
Step 5
Cut remaining 3 sheets of phyllo in half to create 6 short rectangles. Place a rectangle in the center on top of sugared phyllo; brush with butter and sprinkle with cinnamon sugar. Repeat with remaining phyllo rectangles, stacking on top of each other. This will reinforce the center of the galette. (Melt more butter if needed.)
Step 6
Spread cream cheese mixture over phyllo rectangles. Using a slotted spoon or your hands, pile fruit mixture on top of cream cheese; discard any liquid left behind in the bowl. Fold phyllo up and over fruit, pleating and ruffling as needed. Brush outer edges with remaining butter.
Step 7
Bake galette, rotating pan halfway through, until phyllo is deep golden brown and fruit is tender and juicy, 50–60 minutes. Transfer pan to a wire rack.
Step 8
Mix ¼ cup apricot jam and 2 Tbsp. water in another small bowl until smooth. Brush over fruit mixture while still warm. Let galette cool. Serve with vanilla ice cream or whipped cream if desired.