Place shrimp in a small bowl. Dust with cayenne, salt, pepper and thyme. Toss and let sit for 10-15 minutes.
Add pasta to boiling water, add salt. Cook to al dente.
Melt butter in large skillet with the olive oil over medium heat.
Saute onion until soft (3-4 minutes).
Add garlic & red pepper flakes. Stir for 30 seconds (do not let garlic burn).
Add tomato paste, heavy cream, Parmesan and a small pinch of sugar. Stir and cook until blended.
Add the shrimp and cook for 2 minutes.
Add the pasta and toss; add reserved pasta water if it seems too dry.
Allow the shrimp to finish cooking (1-2 minutes). Do not overcook.
Add the parsley and squeeze the lemon over.
Nutrition Info Per Serving: Calories 799.2 Total Fat 29.9 g Saturated Fat 13.6 g Polyunsaturated Fat 2.2 g Monounsaturated Fat 10.8 g Cholesterol 284.5 mg Sodium 584.9 mg Potassium 486.5 mg Total Carbohydrate 92.6 g Dietary Fiber 5.5 g Sugars 6.1 g Protein