To make peach puree, pulse peeled and pitted peaches in a food processor or blender until completely smooth. Transfer to a saucepan set over medium heat. Whisk together sugar and cornstarch in a small bowl, then whisk in to peach puree along with lemon juice. Bring to a boil, stirring occasionally (it will spit quite a bit so be cautious). Let boil for 2 minutes or until noticeably thickened. Remove from heat and let cool completely. Peach puree can be made ahead of time and refrigerated in a jar or airtight container for up to a week.

Preheat oven to 350 degrees F. Butter a 9-inch square pan; line with parchment paper, leaving an overhang over two of the edges.

Combine pecans, cookies and salt in a food processor and pulse until very finely ground. Drizzle over melted butter and pulse until evenly moistened. Press into bottom of prepared pan. Bake crust for 8 to 10 minutes or until set, then remove from oven and let cool completely.

Reduce oven temperature to 325 degrees F.

In a food processor or the bowl of a stand mixer, pulse room temperature cream cheese, crème fraîche, sugar and flour until smooth. Add eggs, one at a time, mixing well after each addition and scraping down the sides of the bowl as necessary to be sure all ingredients are evenly incorporated. Add lemon juice, vanilla and salt and pulse once or twice more.

Pour batter into cooled crust, spreading into an even layer. Drop dollops of the peach puree on top of the cheesecake batter (you won't use all the puree, save the rest for another use). Use a toothpick or bamboo skewer to swirl the puree into the batter.

Bake for 25 to 35 minutes or until the center of cheesecake is set but still barely jiggly. Let cool on a wire rack to room temperature, then refrigerate at least 2 hours or overnight until ready to serve. Gently use the parchment overhang to lift the entire slab out of the pan. Place on a cutting board and slice into squares (use a large chef's knife, run it under hot water to warm it up and wipe it clean between each slice). Serve chilled or at room temperature. Bars will keep in an airtight container in the refrigerator for up to 5 days.