Preheat oven to 200ºC (180ºC fan assisted oven)/400ºF/Gas 6.
Cut the beetroot into wedges (approx 1cm thick), place on a baking tray, toss with 1 tblsp olive oil and roast for 15-20minutes, until tender and the edges are starting to turn golden.
Unroll the pastry and line a deep 22cm tart tin – don’t trim off the excess pastry, as it will shrink a little when it cooks. Line with baking paper, fill with baking beans and bake blind for 15 mins.
Remove the beans and paper and return the pastry to the oven for 5 minutes.
Meanwhile, melt the butter in a small sauté pan, add the spring onions and cook over a medium heat for 5 minutes and set aside.
Turn down the oven to 160C/140C fan/ gas 3.
Trim the edges of the pastry case with a sharp serrated knife and sit on baking tray.
Arrange the roasted beetroot, spring onions and mackerel in the bottom of the pastry case. Whisk the dill, eggs, milk and cream and season well. Carefully pour over the beetroot and fish and bake for 35-40 mins, or until set with a slight wobble in the centre. Allow to stand for 10 minutes before serving. Delicious served warm or cold!