Heat oven to 180C/fan 160C/gas 3. Blitz the flour, butter and cheddar in a food processor until the mixture resembles breadcrumbs, then add one lightly beaten egg and whizz again until it forms a ball. Use the pastry to line 4 individual tart cases, then chill for 10 mins. Line the pastry cases with greaseproof paper, fill with baking beans and bake for 7 -10 mins until the pastry feels dry. Remove the paper and baking beans and continue to cook for a further 2 - 3 mins until lightly golden, then remove from the oven.
Make the custard by mixing the remaining eggs into the Dijon mustard and crème fraiche. Scatter the halved cherry tomatoes on the base of the tarts, sprinkle over the thyme leaves and pour over the custard. Bake for 10 mins, turn the oven down to 150C/fan 130C/gas 3, then bake for a further 15 mins until the custard is just set. Cool slightly before serving.