Preheat the fan oven to 180°C (400°F/gas 6). Heat the oil in a casserole dish (Dutch oven) over a medium heat and soften the onions, squash and thyme for 5 minutes.
Add the garlic and celery and cook for 10 minutes, stirring occasionally, then cover with a lid and leave to cook gently for a further 10 minutes.
Add the passata, water, lemon juice, Worcestershire sauce, horseradish sauce and some Tabasco. Stir, increase the heat and bring to the boil before placing the pan in the oven and roasting, uncovered, for 50 minutes.
Add the olives, stir, then return the pan to the oven for a final 15 minutes. Serve immediately, with rice, baked potatoes or mash and some bitter green leaves.