Preheat oven to 375°F.
Prepare the peach filling: In a bowl place peaches, lemon juice, sugar, thyme, and cornstarch. Mix well and place in the refrigerator.
Prepare the cheese filling: In a separate bowl, mix with a small hand blender the Neufchatel cheese with milk and the egg and place in the refrigerator for about 30 minutes until thick. (Be sure to have all of these ingredients at room temperature before combining.)
For the dough: In a food processor place flour, salt, sugar and vinegar, pulse until combined. Add butter and beaten egg and continue to pulse until mixture forms a ball. Remove from the food processor and flatten out on a floured surface with the palm of your hand. Form a round disk, wrap in plastic and refrigerate for one hour or overnight if you want to prep this step in advance.
Roll out the dough on a floured surface to create a 12" circle. Roll up the dough on a rolling pin and carefully transfer to a baking sheet.
With a spatula spread chilled cheese and egg mixture around the circle leaving a 2" border.
Scoop peach mixture on top of the cheese egg mixture and using both hands bring up dough towards peaches crimping overlapping the dough. Also, make sure you pinch well where the dough pieces touch each other to form a crimp.
Brush the edge of the crust with the egg wash. Sprinkle the course sugar around the edge of the egg wash where it will stick and sprinkle the peaches with brown sugar.
Bake for about 30 minutes or until golden brown around the edges.
After the crostata has cooled and set, heat the apple jelly in the microwave until slightly melted and lightly brush on the peaches only. This will keep them moist and shiny looking.