1. Boil potatoes in salted water until tender. Drain, then mash with butter, milk, salt, and pepper.
2. Pan-fry chicken sausages in olive oil over medium heat until browned and cooked through. Set aside.
3. In the same pan, sauté sliced onions until caramelized. Add flour and cook for 1 minute. Gradually whisk in chicken stock, simmering until thickened to create gravy.
4. In a small pan, fry sage leaves in butter until crisp.
5. Plate mashed potatoes, top with sausages, pour onion gravy, and garnish with fried sage.