In a Dutch oven or similar large sturdy pot, melt the butter on medium heat. Add the chopped onion, pressed garlic, chopped ginger, cumin and black pepper and cook, stirring occasionally, for five minutes.
Add the squash and potato and mix thoroughly with the onion mixture to coat the vegetables. Add the stock, tomato paste and lemon juice and stir to combine. Bring the soup to a boil, then turn heat down. Cover the pot and simmer for 20 to 30 minutes - just until the vegetables are tender.
Remove from the heat and allow the soup to cool some before processing. Working in small batches, puree the soup in a stand blender, pouring the pureed soup into another pot as you go. Alternatively, you can use an immersion blender and puree the soup in the pot.
Once it is smooth, return the soup to the heat and stir in the cream. Season to taste with salt and pepper. Warm through and serve immediately.