Put potatoes, milk, water, salt and pepper into the Thermomix bowl and cook with the measuring cup (MC) off for 12 minutes | 100C | Speed 1
Put carrots into the Varoma bowl and put fish on the top tray. After the Thermomix stops, remove the MC, put the Varoma on top and cook for 10 minutes | Varoma | Speed 1
In the meantime, in a small saucepan, heat 2 tbsp light olive oil on medium-high heat, then add flour and stir to combine.
Slowly add milk and whisk to combine. Reduce heat and whisk occasionally until the mixture thickens. Do not let it boil
Once the Thermomix stops, remove the Varoma and put the butterfly into the bowl
Add mustard and then mash with MC on for 20 seconds/Speed 3
To finish the filling, heat olive oil in a large frying pan on medium-high heat, add onions and garlic and cook for 1-2 minutes until the onions are soft
Add peas and carrots into the frying pan, reduce heat to medium-low, add the white sauce and stir to combine
Remove any skin on the fish and break up into approximately 3-4cm pieces and add to the frying pan. Stir gently to combine
Turn on the grill in the oven to heat up
Put filling into a rectangular baking dish and cover with the mashed potato
Score the mash potato with a fork, sprinkle with grated cheese and put in the middle shelf of the oven.
Grill for approximately 10-15 minutes or until top is browned