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Heat oven to 200C/180C fan/gas 6. Rub the beetroots and garlic cloves with 2 tsp olive oil. Wrap in a double layer of foil, adding 5 tbsp water into the package. Place in an ovenproof dish and bake for 2 hrs or until tender. Test them by inserting a small sharp knife. When the beetroot is cool enough to handle, gently rub off the skins. Set aside, saving the garlic cloves.
Meanwhile, heat a small frying pan. Toast the coriander seeds for a few mins until aromatic, then grind using a pestle and mortar. Toast the cumin seeds, leave them whole and mix in with the coriander.
Heat 3 tbsp olive oil in a frying pan, add the onions, toasted spices and some seasoning. Cook very gently for 30 mins or until the onions are soft and golden.
Unroll the pastry and cut out 4 large circles using a 12cm cookie cutter (or use a small sharp knife to score round a bowl or plate). Place the pastry circles on a baking- parchment-lined baking sheet. Gently score a 1cm border around each and prick the centre area with a fork. Bake in the oven for 10-15 mins. Meanwhile, mix the harissa, vinegar and remaining 1 tbsp olive oil in a small bowl. Thinly slice the beetroot. Remove the pastry bases from the oven and carefully press down the middle areas with the back of a spoon to flatten.
Squeeze the garlic cloves out of their skins and stir into the onions. Spread this mixture evenly into the tarts, then layer on the beetroot slices in a circular pattern, slightly overlapping them. Brush the harissa mix onto the beetroot, then brush the egg on the border and sprinkle with the nigella seeds. Bake in the oven for a further 12-15 mins until the pastry is cooked and golden.
To make the salad, whisk together the extra virgin olive oil, tahini, lemon juice, honey, seasoning and 1 tbsp water. Toss the chicory and watercress in the dressing. Remove the tart from the oven, scatter over the feta and serve alongside the leaves.