Preheat the oven to 350°F. Line a baking sheet with parchment paper and set aside.
Peel and mash the bananas in a large bowl until smooth with only a few small lumps remaining. Hull and slice the strawberries into small pieces.
Add the rolled oats, walnut butter, chia seeds, and sliced strawberries to the mashed banana. Stir until everything is fully combined and no dry oats remain. The mixture will be thick and slightly sticky.
Using a large spoon or cookie scoop, divide the mixture into 12 equal portions. Roll each one into a ball between your palms and place on the prepared baking sheet. Press each ball down gently into a round cookie shape about half an inch thick.
Place the dark chocolate chips on top of each cookie, pressing them in lightly so they stay in place during baking.
Bake for 12 to 15 minutes until the edges are set and the tops look dry and lightly golden. Allow to cool on the baking sheet for 10 minutes before moving them. They firm up as they cool so give them the full time before handling.