125g chilled butter, cut into cubes (plus extra for greasing the tin)
For the pastry, combine the flours, salt and butter in a food processor and process until they resemble coarse breadcrumbs. Add 1-2 tbsp of cold water a little at a time and pulse until the mixture comes together in a mass. Lightly knead to form a consistent dough, wrap in cling film and refrigerate for 20 minutes.
Heat your oven to 220°C. Roll the pastry to a 5mm thickness and use it to line a greased 22cm fluted tart tin about 3 1/2cm high. Refrigerate for 5 minutes to firm, then blind bake the tart case, unweighted, for 15-20 minutes until it starts to brown. Remove from the oven and set the tart case aside. Turn down the oven to 180°C.
In a saucepan over medium heat, melt the butter and fry the onions and leeks for about 5 minutes until well softened but not colored. Remove from the heat. Stir through the salt, allspice, thyme, milk and beaten eggs. Pour into the pie case and bake for about 30 minutes until the tart is set in the centre. Remove from the oven and allow to cool to room temperature.
Place the sour cream into a piping bag with a small nozzle and dot the surface of the tart. Scatter with the salmon roe and chives, then slice to serve.
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