Heat a large skillet over medium-high heat. Melt 3 tablespoons of butter and add in the onions and garlic. Cook for about 5 minutes or until the onions are translucent. Add the shrimp, season and cook for two minutes on each side or until pink and cooked through. Reduce the heat to medium-low, add in the cream and bring to a gentle simmer, stirring occasionally. Season with salt and pepper to taste and reduce for about 5 minutes. Add in the Parmesan cheese and spinach leaves, allowing the spinach to wilt in the sauce.
Bring a large pot of water to a boil and season with a large pinch of salt. Add the linguine and cook until al dente or to package instructions, about 12 minutes.
Pour the shrimp over a large bowl of cooked linguine, sprinkle with parsley and serve.