1Peel and dice the potatoes into small cubes.
Clean the leeks thoroughly and slice them thinly.
Chop the white onion finely.
Mince the garlic cloves.
In a large pot, heat the butter and olive oil over medium heat.
Add the chopped onion and garlic to the pot and sauté until the onion becomes translucent.
Add the sliced leeks to the pot and cook for about 5 minutes, stirring occasionally.
Add the diced potatoes, thyme, and rosemary to the pot and stir well.
Pour in the chicken stock and bring the mixture to a boil.
Reduce the heat to low, cover the pot, and let it simmer for about 20-25 minutes, or until the potatoes are tender.
Season the soup with salt and freshly ground pepper to taste.
Use an immersion blender to blend the soup until smooth, or leave it chunky if you prefer.
Serve hot, garnished with additional herbs if desired.