Place a 4-quart saucepan or Dutch oven over medium heat. Add the 2 tablespoons butter and let it melt.

Add the diced yellow onion and cook for 5–7 minutes, stirring occasionally, until soft and translucent with lightly golden edges. If the onion starts browning too quickly, lower the heat to medium-low.

Stir in the 3 minced garlic cloves and cook for about 30 seconds, just until fragrant. Do not let the garlic become dark brown, as it can turn bitter.

Add the 4 cups pumpkin puree, 4 cups broth, 1 teaspoon cinnamon, 1/4 teaspoon nutmeg, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Stir until evenly combined.

Bring the soup to a gentle simmer over medium-high heat, then reduce the heat to medium-low. Simmer uncovered for 20 minutes, stirring occasionally. The soup should be hot and gently bubbling, with the onions completely tender.

Turn off the heat and use an immersion blender to puree the soup directly in the pot until smooth and creamy, about 1–2 minutes. Move the blender around the pot to eliminate any remaining pieces of onion. If using a countertop blender, blend in batches and leave the vent open while covering it loosely with a towel to allow steam to escape.

Return the blended soup to low heat if necessary. Stir in the 1/2 cup heavy cream and warm for 2–3 minutes, stirring frequently. Do not bring the soup to a rolling boil after adding the cream.

Taste and adjust the seasoning if needed: Ladle the soup into bowls and garnish with toasted pumpkin seeds, a drizzle of heavy cream, or chopped parsley, if using. Serve warm.