Heat oven to 180C/160C fan/gas 4 and line a baking sheet with parchment. Unroll the pastry onto the baking sheet. Spread the chutney over the surface, leaving a border of about 1cm free around the edges. Fold in the edges and roughly pinch them together at the corners to make a raised border.
In a bowl, whisk the crème fraîche, cheese and eggs with a little salt and a good pinch of pepper. Pour the mixture into the centre of the tart, easing it out towards the edges with the back of a spoon. Top with tomatoes, cut-side facing up, and ruffles of prosciutto.
Bake in the oven for 35-40 mins, until the pastry is golden and the filling has set. Leave to cool a little before scattering over the lamb’s lettuce, or leave to cool completely and store in the fridge. Lovely for lunch the next day.