For the pastry, measure the flour and butter into a food processor and whizz until the mixture is like breadcrumbs. Alternatively, place the dry ingredients in a mixing bowl and rub in the butter with your fingertips. Add the egg and whizz again until a ball of dough is formed.
Sprinkle your work surface with flour and roll out the dough to the thickness of a £1 coin and large enough to fit into a 12 x 36cm loose-bottomed tranche tin with 2.5cm-3cm sides. Line the base and sides with the pastry, leaving a generous edge to allow for shrinkage in the oven, prick the pastry all over with a fork and chill in the fridge for 30 minutes.
Heat the oven to 200C/fan 180C/gas 6 and place a large baking sheet inside to get very hot.
For the filling, break the eggs into a jug or bowl, add the creme fraiche, Cheddar, Parmesan and half of the marjoram. Season with salt and pepper and mix until well combined.
Line the pastry case with baking paper and baking beans, place it on the hot baking sheet and and bake blind for 15 minutes. Remove the paper and beans and return to the oven for a further 5 minutes to dry out. Remove the pastry case from the oven and lower the oven temprature to 180C/fan 160C/ gas 4.
Pour the filling into the pastry case and lay the tomato slices overlapping in 5 or 6 rows widthways across the top. Arrange the olive halves in between the rows of tomatoes and sprinkle with the remaining marjoram. Brush the tomatoes and olives with the balsamic vinegar.
Bake for 25 - 30 minutes until the pastry is golden and cooked and the top is browned. Trim the edges to remove any overhanging pastry, then carefully removed from the tin and serve warm with dressed salad leaves.