Preheat the oven to 180C/350F/Gas4.
For the pastry: place the flour, butter and salt in a food processor and pulse until the butter is incorporated. Add the egg and process into a ball. Roll and line a buttered tart tin and refrigerate.
For the filling: chop the bacon into small pieces and fry off in a little oil until slightly crisp. (Reserve a third of the bacon to go into the bechamel sauce.)
Blanch the baby spinach in boiling, salted water for 1 minute until wilted. Drain and squeeze out any excess liquid.
Scald the milk with the shallot and leave to infuse for 10 minutes.
For the bechamel sauce: melt the butter in a medium saucepan, add the flour and cook to a foam over a low heat. Add seasoning and whisk in the milk (discarding the shallot) until the sauce thickens. Remove the pan from the heat and add the egg yolks, grated parmesan and the remaining bacon pieces and beat together.
Bake the tart case blind for 5 minutes or until the surface of the pastry looks dry. Remove from the oven and spread the spinach and remaining 2/3 of the bacon across the base evenly.
Add a pinch of salt to the egg whites and use a hand whisk to bring to stiff peaks. Fold the bechamel sauce and cubes of blue cheese into the whites with a metal spoon using a cutting/folding action. Check the seasoning.
Spread the mixture over the spinach and bacon and return to the oven for 20-25 minutes until risen and golden.
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