Put the flour and a pinch of salt in a large bowl. Rub in the shortening until the mixture looks like breadcrumbs. Stir in the egg and about 1-2 tbsp water to form a soft dough. Knead well then wrap and chill for at least 30 minutes.
Preheat the oven to 200C/gas 6/fan 180C. Knead the pastry again and roll it out to line a 20cm loose-bottomed flan tin or individual tart tins (the pastry won’t shrink, so no need to chill). Patch holes with spare pastry and brush the base and sides with egg.
Make the custard by mixing the remaining eggs into the Dijon mustard and lactose free cream. Ssprinkle over the thyme leaves, cheggar and pour over the custard. Arrange slices of tomato on the top. 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.