Put the chicken breasts between two sheets of baking parchment and, using a rolling pin, roll to a 1cm thickness. Rub with the paprika, lemon zest, ½ tbsp oil, salt and pepper. Cover and set aside while you start the lentils.
Heat the remaining oil in a pan and add the garlic, fennel seeds and fennel slices with a pinch of salt. Cook for 5 mins until starting to soften, then tip in the tomatoes, vinegar and lentils. Turn down the heat and leave to bubble away.
Heat another non-stick frying pan over a high heat, add the chicken breasts and cook for 7 mins, turning halfway, until golden brown and cooked through. Add the watercress to the lentils and stir to wilt, then squeeze in the lemon juice. Slice the chicken then serve it alongside the lentils with a dollop of yogurt and dill.