The earthiness of these black and white beans is complemented by a broth infused with the spices and aromatics that make Vietnamese phở so distinctive.
Soak the beans in salted water overnight. 10 g salt per litre of soaking water. The black beans should be soaked separately from the white beans, as we are going to cook them separately.
On the day
Add the 3 liters of water to a large saucepan. Add the kombu and dried shiitake mushrooms. Bring to nearly a boil, then remove from the heat and let sit for 1 hour. This makes a kombu dashi.
Peel and halve the onions.
Wash, halve, and depth the red pepper.
Wash and halve the carrots lengthways.
Wash the ginger and cut through horizontally to expose all the ginger inside.
Halve the head of garlic and remove any loose skin.
Under a broiler, or better still, over a gas grill, char the onions, ginger, and red pepper. A blast with a blow torch might help with the ginger.
In a skillet, lightly toast the star anise, cloves, and cinnamon stick. Transfer to a stainless steel spice infuser or muslin bag.
Cook the black beans
Drain soaking liquid from the black beans and transfer the beans to a saucepan.
Add 1.5 liters of water, two halves of a charred onion, two halves of the head of garlic, and a bay leaf.
Bring to a boil then reduce to a simmer. Simmer for 1 hour.
Cook the white beans in the broth
After the kombu and shiitake mushrooms have been soaking in the 3 liters of water for an hour, remove the kombu and discard.
Drain soaking liquid from the white beans and transfer the beans to the large saucepan with the kombu dashi.
Add the red pepper, the charred ginger, the halved carrots, the rock sugar, the spices in the infuser, and a bay leaf.
Put the remaining two halves of a charred onion in a larger stainless steel infuser and add to the saucepan. Keeping it in the infuser will make it easier to retrieve at the end.
Bring to a boil then reduce to a simmer. Simmer for 1 hour.
Once the two pots of beans have been simmering for 1 hour, add 2 tsp of salt to each of them.
Simmer for another hour, or until the beans are of edible consistency. Smaller beans take less time.
Remove the onion, garlic, and bay leaf from the pot of black beans. Drain and rinse the black beans under running water.
Remove the red peppers, carrots, ginger, bay leaf, shiitake mushrooms, and infusers from the pot of white beans.
Add the black beans to the pot of white beans and broth.
Add the tsp of black pepper and the soy sauce. Check seasoning and adjust if necessary.
Ladle into bowls and serve with chilli oil and fresh mint.