Sift the flour into a mixing bowl and and cut in the butter, rub until the mixture resembles fine breadcrumbs. Mix in the egg and combine all ingredients. With your hands form a ball, wrap in cling film and chill in the fridge for at least 30 minutes.
Preheat oven to 160 C / Gas 3. Grease a tart tin.
Remove the pastry from the fridge and roll the pastry out on a floured surface. Line the bottom and sides of the prepared tart tin. Prick the pastry with a fork. (It is helpful to line the pastry case with greaseproof paper and fill with dried beans for blind baking. Discard both after use).
Bake for 15 minutes. Remove from the oven.
Meanwhile, cook the spinach in 80ml water until it releases liquid. Slice chilled Brie cheese. Place the spinach onto the baked pastry case. Arrange Brie slices on top and sprinkle with peanuts. Season with salt and pepper.
Bake until the pastry is golden brown and the cheese melts, about 20 minutes. Remove from the oven and serve.