Warm olive oil in a skillet over medium heat. Add onion, bell pepper, and ¼ teaspoon salt. Cook 6–8 minutes until tender.
Stir in garlic, oregano, tarragon, and cinnamon. Cook for 1 minute until fragrant.
Add tomatoes and cook 1 minute, stirring. Stir in pumpkin purée. Simmer 5 minutes, then remove from heat.
Transfer mixture to a blender. Add 1 tablespoon butter and 1 teaspoon vinegar. Blend until smooth and creamy.
Season with black pepper and more salt if needed. Adjust butter/vinegar for richness or tang. Blend again.
Toss with warm pasta and serve topped with parmesan and parsley, if desired.
