Melt the butter and heat the olive oil in a large pot. Stir in the shallots, leeks, garlic, salt and pepper and sauté for 3 minutes.

Stir in the potatoes and cook for 5 minutes, stirring occasionally.

Stir in the milk, stock and thyme and bring to a boil.

Reduce the heat to a simmer and cook for 30 minutes, stirring often.

Remove from the heat and discard the thyme stems.

Using an immersion blender, carefully purée the soup until smooth.

Ladle the soup into serving bowls and garnish with the parsley and Costa Azul, if desired.