Whilst the beans are cooking, heat the oil in a large oven-proof pot with a lid.
Add the onions to the pot, and stir to cover the onions with oil.
Reduce the heat to low, cover the pot, and allow the onions to slowly cook for around 45 minutes. Stir the pot from time to time to prevent the onions catching on the bottom. When ready the onions should be soft and golden in colour.
Pre-heat the oven to 170C (150C fan forced).
Add the mustard, molasses, sugar, Worcestershire sauce and cloves to the onions, and stir to combine. If making the vegetarian version, add the smoked paprika at this stage.
Stir in the tomato passata and water, season with salt and pepper, and mix thoroughly.
Gently stir in the cooked beans. If using the ham hock, add this to the pot and gently push the hock to the bottom of the pot so that it is covered by liquid.
Cover the pot, and place in the oven for approximately two hours, or until the liquid in the pot has thickened, and the meat on the hock (if using) can be easily pulled from the bone. [See Note 5 below]
Turn the oven off, and remove the hock from the beans.
Shred the meat from the hock and gently stir the shredded meat back into the beans.
Taste and adjust the seasonings if required.
Serve with well buttered toast.