Boil the heerenbone in unsalted water for about 40 minutes until completely tender, then drain.
Purée the cooked beans with the butter, olive oil, lemon juice and salt in a food processor until completely smooth.
To make the mussels
In a large pot, heat a splash of olive oil to smoking point. Add the mussels all at once and cover with a lid.
Cook for 1 minute, then lift the lid and stir to evenly distribute heat.
Add the white wine, cover and cook for a further 2 minutes until all the shells have opened. Strain, retaining the mussel stock.
Remove the cooked mussels from their shells and double-check that all beards are removed.
To serve
Heat the mussels in a little of their own stock. Add the remaining stock to the heerenbone purée and heat through.
Flash fry the dune spinach leaves in a bit of olive oil and a squeeze of fresh lemon juice.
Scoop the purée into heated bowls, top with mussels and dune spinach leaves, and serve immediately.
* Heerenbone (English translation: “The Lord’s bean”): A South African heirloom bean cultivated in the Sandveld region of the West Coast of South Africa. Substitute with butter beans or lima beans.** Dune spinach: Native to the dunes of South Africa. Substitute with baby spinach or New Zealand spinach.