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 the egg and whizz again until it forms a ball.
Lightly dust a cool surface with flour and roll out the pastry. Line a 24cm loose-bottom tart tin with the pastry, then chill for 10 mins. Line the pastry case with greaseproof paper, fill with baking beans and bake for 15 mins. Remove the paper and baking beans and continue to cook for a further 10 mins, then remove from the oven.
Mix together the Dijon mustard and crème fraîche and spread evenly over the pastry base. Halve the tomatoes, remove the stalk and arrange in the pastry case. Sprinkle with thyme and freshly ground black pepper. Return the tart to the middle of the oven and cook for 30 mins. Reduce the heat to 150C/fan 130C/gas 2 and cook for a further 30 mins. Serve warm or at room temperature.