Heat your oven to 180°C/Fan 170°C/Gas 4. Tip the flour into a bowl, saving 1 tbsp for later. Coarsely grate in the butter. Add a good pinch of salt and pepper. Rub the butter into the flour till you have what resembles fine bread crumbs.
Separate the egg. Add the yolk to the flour. Save the egg white in a separate bowl. Add 2 tbsp cold water and bring the dough together. Add a little more water if you need to but take care not to make the dough too wet. Wrap the dough in cling film and pop into the fridge to rest.
Peel and thinly slice the onions and garlic. Heat a frying pan for a couple of mins. When warm, drizzle in 1 tbsp olive oil. Add the onions and garlic and cook on a low heat for 15 mins.
While the onions cook, put the porcini mushrooms into a jug and pour over 200ml boiling water. Leave to soak for 5 mins. Drain and reserve the porcini infused water. Finely chop the porcini. Finely chop the rosemary leaves. Zest the lemon.
When the onions have cooked for 15 mins, add the chopped porcini and they're soaking liquid. Add the rosemary. Cook together for 5 mins on a high heat till the liquid has evaporated.
While the porcini and onion mix cook, Stir the lemon zest through half the pot of mascarpone. Add a good crack of black pepper. Remove the dough from the fridge. Dust your surface with the reserved flour. Roll the pastry into a large circle, approximately the size of a dinner plate.
Place the dough on a baking tray lined with baking paper. Spread 1/4 of the pot of mascarpone over the base of the tart, leaving a 2 cm border. Top with the onion and porcini mix. Dot over another 1/4 of the tub of mascarpone. Place 6 or so tomatoes on top. Scatter over plenty of black pepper. Fold the edges over the filling. It won't enclose the filling. It acts as a barrier to stop the filling escaping.
Beat the egg white and brush it all over the pastry. Bake for 25 mins till the pastry is golden.
Juice the lemon. Whisk with 1/2 tbsp oil and some seasoning. Half the remaining tomatoes. Toss the dressing through the rocket and tomatoes. Serve with wedges of the porcini and mascarpone tart.