Preheat oven to 160°C (180°C for non-fan ovens)
Roughly chop walnut pieces and combine with rolled oats, chia seeds, mixed seeds, black seeds, and crispy onion pieces in a bowl
Add black pepper and salt to the mixture
Prepare instant soup according to package directions using boiling water (approximately 300ml total liquid)
Mix the soup into the dry ingredients using a fork and spatula until well combined
Line two baking trays with parchment paper and divide the mixture between them
Flatten the mixture into an even layer and compress firmly using a spatula and potato masher
Cut the mixture into 16 bars (approximately 5.5cm x 8cm each) while still in the tin
Bake for 44-56 minutes, checking every 12-15 minutes and flattening if needed
Remove from tins and place directly on oven racks to finish crisping
Bake for an additional 12 minutes until completely crisp
Cool completely on racks (approximately 45 minutes) before serving
