Heat the butter and oil together in a large pan and add the onions, leeks and garlic with a large pinch of salt. Sauté the vegetables gently until softened and fragrant.
Add the potatoes to the pan and gently cook for a few more minutes.
Crumble the stock cubes into the pan and add enough water to cover the vegetables by 2 inches or so. Season well with salt and pepper, cover with a lid and leave to simmer for 20 minutes or until the potatoes are very soft.
Remove the lid from the pan after this time and add the watercress. Replace the lid and cook for 5 minutes more so that the watercress is cooked but still a vibrant green.
Add the cream to the soup and blend, using an immersion blender, until very smooth.
Return the pan to the heat and warm the soup until piping hot. If it is too thick, add some more water or cream until you have the consistency you prefer.
Serve in warmed bowls topped with a drizzle of cream, and croutons or crusty bread.
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