To make the pastry: combine the flour, butter, salt and egg yolk by pulsing ingredients together in a food processor, taking care not to over-process, until it forms into a ball (add up to 1 tablespoon of water if it seems to be too dry).
On a floured surface roll out the pastry to a circle big enough to fit the base and up the sides of a loose bottom 9 inch tart tin; lift pastry into the tart tin, smoothing the bottom and pushing the pastry well into the sides; brush any thin patches of pastry with egg white to seal; chill pastry case in the fridge for at least 20 minutes.
Preheat the oven to 180C, fan 160C, gas mark 4.
Meanwhile put the garlic cloves, unpeeled, into a small saucepan of boiling water and simmer for 10 minutes; drain them then place on a small baking dish and drizzle with a little olive oil.
Remove the tart tin from the fridge and cover the pastry with baking beans, then place on a baking sheet and bake for 12 minutes along with the garlic cloves (which will need an extra few minutes in the oven until the skins are golden and the flesh is soft).
Remove the baking beans from the tart cast and make certain the pastry is dry to the touch ( if not, put it back in the oven for another minute or two), if there are any cracks or tears, repair them with a little raw pastry and beaten egg.
Squeeze the roasted garlic out if its skin, mash to a puree with the back of a spoon and spread thinly over the base of the cooked pastry case.
To make the filling: mix the eggs, cream and milk together with a whisk, then season; cut the goats’ cheese into slices, lay them on top of the garlic puree, scatter over the thyme leaves then pour in the custard.
Bake for 25 - 30 minutes until golden and lightly set; serve warm.