Place large pot on stovetop over medium heat. Add 2 tablespoons unsalted butter to pot and let butter melt completely, pushing butter across pot occasionally to cover entire surface.
When butter has melted, add 1 cup chopped yellow onion, ½ cup chopped celery, and 2 teaspoons minced garlic to pot. Stir to incorporate. Sauté vegetables, stirring occasionally, approximately 5 minutes or until onions have softened.
Once onions have softened, add 2 teaspoons curry powder, 1 pound cubed sweet potatoes, and 1 cup chopped Granny Smith apples. Stir to incorporate. Sauté potatoes and apples, stirring occasionally, 2 minutes.
After 2 minutes, pour in 1 quart low-sodium vegetable stock and stir well.
Reduce heat under pot to low. Place lid on pot and let mixture simmer 20 minutes or until sweet potatoes can be pierced easily with tines of fork.
When sweet potatoes are cooked through, remove pot from heat. Let mixture cool slightly, then insert immersion blender into pot. Blend mixture until completely smooth.
Once soup is completely smooth, remove immersion blender. Add ½ cup heavy cream and stir to combine, then return pot to low heat.
Taste soup and season with salt and freshly cracked black pepper to taste. Simmer soup, stirring occasionally, 2 to 3 minutes or until hot.
Portion soup into serving bowls. Top each bowl with drizzle of heavy cream and roasted pumpkin seeds if desired. Serve warm.