Place the parmesan cheese into the Thermomix bowl and grate for 5 seconds, Speed 8. Set aside.
Insert the butterfly and place the sweet potato cubes and milk into the Thermomix bowl. Cook for 25 minutes, 90 degrees, Speed 2. Add the butter and mash for 30 seconds, Speed 3. Remove the butterfly and allow to cool.
Add the parmesan cheese, tuna, eggs, breadcrumbs, salt and pepper. Mix for 5 seconds, REVERSE, Speed 2.
Chill the mixture for 30 minutes (this helps when forming into patties).
Using wet hands, shape the mixture into 12 patties. Press down lightly to flatten.
Heat the olive oil in a large frying pan over medium-high heat.
In batches, cook the patties for 3-4 minutes on each side or until golden brown and thoroughly heated through.