Heat olive oil in a large saucepan over medium heat. Add onion; cook 2 to 3 minutes. Reduce heat to medium-low; add garlic and cook until onion is beginning to soften, 2 to 3 minutes more. Stir in sugar substitute, pumpkin pie spice, and salt; stir in chicken stock to combine. Whisk in pumpkin purée until fully incorporated. Cover; simmer soup for 10 minutes.

Blend soup carefully using an immersion blender until smooth; stir in 1 cup heavy cream until incorporated. Taste; adjust salt, if necessary.

Drizzle servings with heavy cream; sprinkle with walnuts.