Heat 4 tablespoons butter over medium-low heat in a pot and cook onion until soft and translucent, about 5 minutes. Add pumpkin and potatoes and stir to coat with butter. Add broth and milk. Bring to a boil, reduce heat, and simmer, covered, until potatoes soften, 10 to 15 minutes.

Strain soup through a fine-mesh sieve. Season with salt, nutmeg, and sugar. Stir in lemon juice. Return to the pot and bring to a boil. Add cream and heat through but do not boil.

Meanwhile, heat remaining butter in a skillet and toast bread cubes in the hot butter until browned, 3 to 5 minutes. Serve croutons with the soup.