Finely chop silverbeet stalks. Heat olive oil in a large frying pan over medium heat, add stalks and onion, reduce heat to low-medium, stir frequently until tender (8-10 minutes).
Meanwhile, blanch leaves (1-2 minutes), then refresh. Squeeze out excess liquid, coarsely tear, set aside.
Preheat grill to high. Melt butter in a saucepan over medium heat until foamy. Add flour, stirring rapidly until golden (1-2 minutes), then gradually whisk in warm milk. Season to taste with nutmeg, sea salt and freshly ground black pepper. Stir in onion mixture and silverbeet leaves, cook until tender (4-5 minutes). Spoon into a shallow 2 litre-capacity ovenproof dish, scatter with Gruyère and grill until golden (1-2 minutes). Serve with roast loin of Wessex saddleback.