Preheat the oven to 200 degrees C/400 degrees F (180 degrees C/350 degrees F fan).
Put the neck fillets into a dish. Roughly crush the coriander and fennel seeds in a pestle and mortar, then mix with the chile flakes and half the olive oil. Press all over the lamb, season well and set aside for 10 minutes.
Meanwhile, put the tomatoes and beans on a serving platter. Mix the vinegar and lemon juice with some seasoning, then whisk in the extra-virgin olive oil; pour this over the tomatoes and beans.
Heat the remaining tablespoon of olive oil in an ovenproof frying pan and fry the spiced lamb on all sides until seared and golden all over. Transfer to the oven and cook for 8 to 10 minutes (the lamb should still be pink in the middle). Rest for 5 minutes, then slice and gently toss through the beans and tomato salad. Scatter with the basil leaves and serve.