To make potato and celeriac mash, boil, steam or microwave potato and celeriac, separately, until tender. Drain. Place potato and celeriac in large bowl; mash with milk and butter until smooth. Stir in cheese. Season to taste. Cover to keep warm.
Melt butter in medium saucepan. Cook onion, stirring, about 5 minutes or until softened. Add flour and cook, stirring, until mixture thickens and bubbles. Gradually add milk; stir until mixture boils and thickens. Remove from heat; stir in spinach, tuna and juice. Season to taste.
Spoon tuna mixture into shallow flameproof 1.5-litre (6-cup) dish; top with mash. Place under grill until browned lightly.