Spinach Souffle Tart

A slightly tricky tart to master if you haven't made a souffle before and there are 5 separate elements that bring this together. I suggest you read the method throughly so you know exactly what to expect. The hard work and mastering several techniques is worth the fluffy rich reward at the end.

6 large eggs separated
600g maris piper potato
100g butter plus extra for greasing
100g plain flour
175ml milk
1 shallot
1 clove garlic
150g grated Gruyere
150g washed baby spinach
1 tablespoon olive oil
Sea salt and pepper
loaf of un-sliced stale white bread

METHOD:
Pre-heat the oven to 160oC. Peel the potatoes and place in a pot of cold water and bring to the boil on a medium to high heat cook till the potatoes just start to fall apart and drain. Leave the potatoes to cool slightly then using the back of a spoon press the potatoes through the sieve and into a bowl.
Grease a fluted edge flan tin with removable base (30cm in diameter and 3cm deep) with butter then dust with flour and tap around the tin dusting all the sides. Cut the crust off the bread loaf and cut into 1cm slices then arrange the slices around the base trim what you can to make a circle shape it doesn't have to be perfect however.
Dice the shallot and crush the garlic then melt the butter in a sauce pan and add the onion and garlic, cook till the onion turns translucent and soft. Add the flour and stir till the flour has absorbed into the butter and  it smells of shortbread then season with salt. Bit by bit add the milk and keep stirring to stop lumps forming, once all the milk has been added cook for a further 3-4 minutes to thicken then remove from heat and add the Gruyere and yolks and stir till evenly incorperated.
Bring a small pot to a medium heat and add the olive oil and spinach with a pinch of salt and gently stir till the spinach has wilted then remove from heat and strain the excess liquid.
Place egg whites in a large mixing bowl and using a electric beater whisk the whites till stiff peeks form then using a large metal spoon gently fold in a third of the cheese sauce and a third of the potato till roughly combined and repeat 2 more times till fully combined. Fold the souffle into the baking tin and evenly arrange the spinach leaves on top and finish with cracked pepper. Place at the bottom of the oven a tray of hot water and place the tart on the middle shelf and bake for 30 minutes.
Once cooked remove gently from oven and leave to rest for about 20 minutes before cutting.