Preheat the oven to 350°F(177°C). Grease 8 inch (20cm) square oven-proof dish and line the bottom and sides with parchment paper, about 2 inches(5 cm) of overhang on each side.
Chop the peaches into almost ½-1 inch (1,25-2,5 cm) pieces. First, mix the peaches with lemon juice and then with the almond extract. Next, toss with cornstarch and sugar. Set aside.
Mix the flour, rolled oats, sugar, cinnamon, salt( if using) well in a medium bowl. Then add the melted butter and stir with a spoon or spatula.
Place ⅔ of the mixture into an oven-proof dish that is lined with parchment paper. Use your hands or a spoon to spread the mixture evenly.
Next, cover the crust with the peach mixture but leave the excess liquid in the bowl. You may want to use this liquid later if there is too much left. ( You can place that liquid into a small pan, and when the mixture thickens you can remove it from the heat and drizzle the sauce on top of the baked bars. )
Sprinkle the remaining dough over the peaches. If you like you can make some big crumble pieces by squeezing crumble mixture in your hand before sprinkling. Use your fingertips to press to the crumbles so they can stick to peaches.
Bake for 33-35 minutes until the top becomes golden brown and the crumbles are crunchy. Let it cool at room temperature for almost 1 hour. Then keep in the fridge for extra 1,5 hours until it is completely cool so you can have nicely cut bars.