Place 1 oven rack in the upper third of the oven and 1 rack in the lower third of the oven. Preheat oven to 190 degrees. Coat 2 large baking trays with cooking spray. On a microwavable plate, spread the walnuts and rolled oats. Microwave on high for 2 minutes. Stir and check for toasting. Microwave for an additional 2 minutes on high and stir. The nuts and oatmeal should now be nicely toasted. Dump in a large bowl.
In the food processor bowl fitted with a steel blade combine the sugars, butter, eggs and vanilla. Process until smooth and creamy, about 30 seconds. In a small bowl, combine the flour, bicarb soda, baking powder and salt. Spoon the flour mixture on top of the batter. Pulse 3 or 4 times to mix.
Scrape the batter into the walnut-oatmeal mixture. Add the raisins and muesli. Fold the ingredients together.
Drop the biscuits onto the sheets using 2 tablespoons of dough for each cookie. Bake in the preheated oven for 12 to 15 minutes, until the tops are just beginning to brown but the middles still look a little soft. Cool on the baking trays for a few minutes and then transfer to a rack.