Preheat oven to 350°F. Line bottom and sides of an 8 x 8 or similar size pan with parchment paper.
To a large bowl or food processor add the flour, oats, brown sugar, salt and butter. Mix or pulse until combined and dough just begins to stick together. Transfer 1/2 of the dough to the baking pan and press down into the pan with the bottom of a measuring cup. Bake in preheated oven for 15 minutes. Allow to cool.
To the remaining dough add 1/2 of the cinnamon and toss to combine. Set aside.
In a mixing bowl, place the diced peaches, sugar, vanilla, remaining cinnamon, cornstarch and lemon juice. Stir until well combined. Scrape on to cooled dough, spreading the peach mixture out to cover the baked dough completely. Sprinkle over the reserved dough, completely covering the the peaches.
Bake in oven for 45 - 55 minutes until browned and bubbling on the sides. Remove from oven and let cool in pan. Remove from the pan using the sides of the parchment paper, cut and serve. For cleaner cutting, chill for at least 1 hour prior to cutting.