Bring the beans to boil in a saucepan of cold water, simmer for 5 minutes, take off heat and cover tightly. Leave for 2 hours.
Drain the beans, return to pan, cover with fresh cold water and add the herb bundle and vegetable quarters. Bring to boil and simmer for 1 hour until tender.
Preheat oven to 200C/Gas 6/fan oven 180C. Heat 3 tablespoons of the oil in a casserole and fry the chopped onion and celery until soft. Add the garlic and cook for 3-4 minutes. Stir in the tomatoes,
sugar and half the tarragon. Season.
Drain beans, retaining 1.2 litres/2 pints of liquid. Discard veg and herbs. Stir 600ml/1 pint of the liquid into the tomato mixture and simmer, half-covered, for 30 minutes. Toss the squash, carrot and celeriac chunks in 5 tablespoons of oil. Season and roast in a tray for 30 minutes.
Remove vegetables and reduce heat to 180C/Gas 4/fan oven 160C. Stir beans, vegetables, mustard and half the parsley into the casserole. Add liquid if needed to make the mixture nicely moist. Check seasoning. Turn into a wide baking dish. (You can prepare to this stage the day before. Cool, cover and refrigerate. Next day, if the beans have absorbed too much liquid, moisten with bean liquid or stock.)
Mix the breadcrumbs with the remaining parsley and tarragon. Scatter over and drizzle with remaining oil. Bake for 50 minutes. (Add an extra 10 minutes if making ahead.) Serve from the dish.