Melt the butter in a large, heavy-based pan and cook the onions slowly, for about 30 minutes, stirring occasionally until soft and caramelised. Set aside to cool.
Beat the eggs, cream and some seasoning together in a medium bowl. Add half the cheese and stir through.
Spread the onion mixture evenly across the base of the pastry shell. Pour in the egg mixture and sprinkle over the remaining cheese. Bake at 180ºC for 20-25 minutes, or until set and golden. Remove from oven and allow to cool.
Combine the walnut pieces with 1 tbsp walnut oil. Toss in a pan over a low heat until lightly coloured (about 1-2 minutes). Set aside to cool.
Make a dressing from the remaining walnut oil, grapeseed oil and vinegar and season to taste.
Pick the tender leaves and sprigs of the watercress. Peel the celeriac and cut into matchsticks, then toss in the lemon juice; core the pears and do the same. Drain the celeriac and pears and place in a large bowl with the watercress, walnuts and dressing. Gently combine.
Cut the tart into wedges. Serve with the salad.