Warm 2 to 3 tablespoons of butter in a small pot.
Add onion and chili flakes and cook until onion is soft (1 to 2 minutes)
Add nectarines and cook for a couple of minutes, until they start to release their juices.
Add the mostarda. Bring to a boil, reduce to a simmer and cook 12 to 15 minutes until the nectarines start to fall apart.
In a large pan, cook sausages for 10 to 12 minutes.
In the same pan, cook the garlic for 30 to 60 seconds (until fragrant).
Add the mustard greens and wilt (2 to 3 minutes)
Slice sausage into rounds.
Serve sausage over greens, with mostarda mix on top.