Step 1

Heat the butter and oil in a large saucepan over medium-low heat. Add the onion. Cook, covered, stirring occasionally, for 20 minutes or until the onion is soft but not coloured.

Step 2

Add potato and stock. Increase heat to high. Bring to the boil. Reduce heat to medium. Cook, partially covered, for 15 minutes or until potato is very soft. Uncover. Set aside to cool slightly.

Step 3

Use a stick blender to blend until smooth. Alternatively, transfer half the potato mixture to the jug of a blender and blend until smooth. Transfer to a clean saucepan. Repeat with remaining potato mixture.

Step 4

Stir in the milk. Place the soup over medium heat and stir until heated through.

Step 5

Ladle the soup among serving bowls. Top with a dollop of sour cream, a teaspoonful of caviar and dill to serve.