Preheat the oven to 200ºC, fan 180ºC, gas 6. Put a baking tray in the oven to heat up. Carefully unroll the pastry and lift into a 28cm x 19cm x 4cm baking dish. Take care not to stretch the pastry, just gently push it into the base and sides. Don’t worry if it hangs over the sides slightly. Prick the base all over with a fork.
Place a sheet of baking parchment on top of the pastry and fill with baking beans or rice. Place on the heated baking tray in the oven and bake for 15 minutes. Remove from the oven and remove the beans or rice.
Crack the eggs into a bowl and add the crème fraîche, grated Cheddar and mustard. Whisk together, then pour half into the pastry case. Lay half the spring onions over the top, then pour over the rest of the mixture. Scatter over the herbs (if using) and arrange the remaining spring onions on top.
Bake for 20-25 minutes, until set but still a little wobbly (it will set further as it cools). Leave the tart to stand for 5 minutes, then slice and serve.