Remove and discard the stems of cavolo nero and silverbeet. Slice leaves into 2cm-thick strips.
In a large, heavy-based saucepan, heat 3 tbsp oil over a low heat. Add onion, garlic and anchovy and cook, stirring until onion and garlic starts to colour. Add stock, silverbeet and cavolo nero and stir to combine. Cover and simmer for 20 minutes.
While the greens are braising, heat a large frying pan on medium-high. When hot, add butter and 1 tbsp oil. Add veal medallions, season with a pinch of salt and fry gently for 2-3 minutes, without turning, until golden-brown on one side.
Turn the medallions over, fry for another 2-3 minutes until golden-brown on both sides. Remove veal from pan, add a grind of pepper and leave to rest in a warm place.
Remove the greens from the heat when they are very tender. The leaves will be a deep green colour. Season with salt and pepper, add lemon juice and mix.
Serve the veal with the braised greens and top with a spoonful of mustard fruits.