Place bacon joint and bouquet of garni in large pan and cover with water. Add the onion and celery and gently bring to the boil (scimming off any scum from the surface with a slotted spoon), reduce heat, cover and simmer for 45 mins.
Remove the meat, cut off any rind and fat and cut the bacon into chunks.
You should still have about 1.5 litres/ 2½ pints stock in the pan. Measure (if you like) adding water to make up the quantity.
Add the potatoes and thyme, bring to the boil, then reduce the heat & simmer uncovered for 10 mins.
Next add the tomatoes and spinach to the pan, season to taste and simmer for a further 5-10 mins until the potatoes are tender.
Put the egg yolk in a large bowl then gradually mix in 2-3 ladles of the soup stock to the egg yolk with a fork. Return the egg yolk mixture to the pan, to thicken soup slightly. Do not return to heat. Remove the bouquet of garni and serve the soup.
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