Pat dry the spare ribs. Wash the thyme, shake dry and pick off the leaves.
Drain the tomatoes, reserving the oil. Coarsely chop the tomatoes and blitz in a blender with the thyme, green pepper, salt, 1 tbsp balsamic vinegar and 5-6 tbsp water to a creamy paste.
Coat the spareribs with the paste, cover with cling film, and leave to marinate in the fridge for at least 30 minutes.
Meanwhile, wash the potatoes and cook in boiling salted water for 20 minutes. Drain the potatoes, rinse under cold water and leave to cool briefly.
Wash and thinly slice the spring onions. Clean and thinly slice the button mushrooms.
Wash the parsley, shake dry and chop the leaves.
Place the spareribs on a baking tray and cook in a preheated oven at 400°F (350°F fan, gas 6) for 10 minutes. Turn them, then cook for another 10 minutes.
Whisk together the stock, remaining balsamic vinegar and 2 tbsp tomato oil. Season with salt and pepper.
Cut the potatoes into slices. Mix carefully with the mushrooms, spring onions, parsley and dressing. Season the potato salad with salt and pepper.
Cut the spare ribs into portions, and serve with the potato salad.