Melt butter in a large saucepan over medium heat.

Saute onions and garlic until softened and translucent, about 5-7 minutes.

Add pumpkin and potatoes to the saucepan and cook for 3-5 minutes, stirring occasionally.

Stir in tomato paste, chicken stock (or vegetable stock), salt, pepper, lemon juice, and allspice.

Bring the mixture to a boil, then reduce heat to low.

Cover the saucepan and simmer gently for about 40 minutes, or until the pumpkin and potatoes are very tender.

Remove the saucepan from the heat and let cool slightly.

Carefully puree the soup in a blender or food processor until smooth.

Return the pureed soup to a clean pan.

Add milk to the soup and bring back to a simmer, whisking continuously.

Simmer for 2-3 minutes, stirring frequently.

Adjust seasonings to taste.

Stir in sour cream until well combined.

Reheat gently before serving.

Garnish with freshly chopped coriander (cilantro) or parsley.