In a large skillet, melt butter over medium heat. Add the onion, garlic, curry paste, ginger, salt, and pepper. Cook, stirring, for 3 minutes or until vegetables have softened. Add the squash, the potatoes, broth, and tomato paste. Let it simmer for 30 minutes.
Remove from heat, add the lemon juice and milk, and mix.
In the food processor or blender, purée the mixture several times.
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