Over medium-low heat. Bring the milk to a simmer and cook the fish until flaky, 5 to 7 minutes; do not overcook.
Remove from heat, drain off the milk and allow to cool. Using a fork, break the fish into large flakes. Add to a large bowl.
In a sauté pan, melt the butter over medium heat. Add the onions and cook until they are softened. Cool and transfer to same bowl.
Add to the same bowl the mashed potatoes, lemon, garlic, dill, savoury, salt, and pepper with one egg. Combine until the savoury/dill and egg have been distributed fully throughout the mixture. Taste and season accordingly.