Preheat the oven to 350℉. Line a 9×9-inch baking pan with parchment paper, leaving about an inch overhang on two sides. Set aside.

Peel and chop the peaches and stir together with the vanilla or lemon zest in a medium mixing bowl, if using. Set aside.

Make the crumble: In a large mixing bowl, stir together the oats, flour, sugars, baking soda, and spices until well-combined.

Cut the butter into cubes, then use a pastry blender or your fingers to gently knead the butter into the dry ingredients, until completely incorporated, and you’ve formed a shaggy, crumbly dough that easily sticks together when pinched. This will take a few minutes.

Assemble the bars: Scoop about two thirds of the crumble dough into the baking pan, and press into an even layer along the bottom. Top with the peach mixture, being careful not to touch the sides of the pan.

Sprinkle the remaining crumble mixture over top, from edge to edge. Gently press the crumbs into the peach layer.

Bake for 35-45 minutes, until the crumble topping is golden brown. The peach juices may be starting to bubble up through the crumbs. Remove from the oven and let cool before slicing into bars. Enjoy!