For the pastry, rub the butter and flour together until breadcrumb consistency. Add the salt and eggs and mix until smooth and well combined. Roll the pastry into a ball, wrap in clingfilm and chill in the fridge for 20 minutes.
Preheat the oven to 150C/300F/Gas 2 and grease a 21cm/10in tart tin.
Roll out the pastry into a large circle. Wrap the pastry around a rolling pin and lift it onto the tart ring. Press gently into the tin, taking care not to stretch the pastry - If it cracks just press it together. (Don’t trim the edges, leave them overhanging.) Cover with clingfilm and chill in the fridge for a further 20 minutes.
Place the tart tin onto a baking tray and cover the centre with greaseproof paper. Fill with baking beans and bake in the oven for 30 minutes.
Remove the baking beans and greaseproof paper from the tart tin. Brush the pastry with egg wash and cook for a further 10 minutes, or until the pastry is lightly golden-brown.
Increase the oven temperature to 170C/325F/Gas 3.
Meanwhile, for the filling, melt the butter in a large saucepan over a low heat. Add the onions, a pinch of salt and cook until softened. Stir in the marjoram.
Remove the onions and marjoram from the pan and chill in the fridge.
Meanwhile whisk the eggs and crème frâiche together in a bowl until well combined. Season with salt and freshly ground black pepper. Stir in the onions and pour the mixture into the tart tin. Finally, sprinkle over the cheese and bake for about 45 minutes, or until cooked through.
Meanwhile for the vinaigrette, whisk together the olive oil and sherry vinegar. Add the spring onions and season to taste with salt and freshly ground black pepper.
Remove the tart from the oven; trim the edges from the pastry. Serve with the green salad and spring onion vinaigrette.