Preheat your oven to 375 F (190 C). Prepare a square baking dish with cooking spray.
To a medium bowl, add the oats, baking powder, cinnamon and salt. Mix to combine.
To a larger bowl, add the melted butter, milk, brown sugar, liquid sweetener, yogurt, egg and vanilla. Mix well.
Then, add the dry ingredients to the wet ingredients and use a spatula or spoon to mix until just combined. Add the peaches (you can save a few to put on top of the bake) and stir.
Pour the mixture into the prepared baking dish and spread it to the edges of the dish evenly. Add the reserved peaches on top. Bake uncovered for 35-45 minutes, until the oats are firm to the touch and slightly browned on the edges.
Let the oat bake cool down for at least 20 minutes before slicing it into 4 pieces (or any serving size you want). Transfer the pieces to airtight containers and store them in the fridge for up to 5 days, or freezer for up to 6 months!