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First, make the pickled onions. Put the onion in a bowl, then bring the vinegar, sugar, salt, mustard seeds and 50ml water to a boil in a saucepan. Stir, on a medium heat, to dissolve the sugar. Once dissolved, remove from the heat and pour over the onions. Set aside for at least 30 mins.
Meanwhile, make the pastry. Mix the flour with a pinch of sea salt in a bowl. Add the butter and use your fingers to lightly bring it together to a breadcrumb texture. Don’t worry if there are some big chunks, this will help make it light and flaky. Stir in the vodka, then pour over 2 tbsp ice cold water and use a butter knife to bring the dough together. Use your hands to form into a ball. If it needs more liquid, add a dash more iced water. Turn out onto a floured worksurface and knead briefly, until smooth and pliable. Squash it into a disc, then wrap in baking parchment and chill for 30 mins.
Heat the oven to 200C/180C fan/gas 6. Put the potatoes and mint in a saucepan and cover with cold water. Add a big pinch of salt and bring to the boil, then simmer for 10 mins until tender. Turn off the heat and allow the potatoes to cool in the water.
Roll the pastry out until about 5mm in thickness, then transfer to a loose-bottomed 20cm tart tin. Trim the edges with a knife, then chill for 10 mins until firm. Prick the base with a fork, line with crumpled baking parchment and fill with baking beans or rice. Bake for 20 mins.
Remove the baking beans and parchment. Whisk one of the eggs in a bowl, then brush all over the pastry. Bake for 10 mins more, then remove from the oven.
Blitz the remaining eggs and egg wash, the wild garlic, watercress and cream to a purée in a food processor. Transfer to a bowl and stir in the drained ricotta and lemon zest. Add most of the cheese, nutmeg and a pinch of salt. Slice the potatoes into bite-sized pieces and arrange half of them in the tart case. Using a teaspoon, dot in the harissa. Pour over the green egg mixture and nestle in the remaining potatoes. Scatter over the rest of the cheese.
Bake for 35-40 mins. Allow to rest for at least 20 mins before you remove it from the tin and slice. Serve with a watercress salad, if you like, and the pink pickled onions.
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