In a large saucepan over medium heat, melt the butter. Add the onions, including juices, and cook until tender, about 2 minutes. Add the garlic, white pepper, paprika, and salt and freshly ground black pepper, to taste. Cook for a few minutes to let the flavors bloom, and then slowly stir in the heavy cream. Bring to a simmer, stirring occasionally, and cook until slightly thickened, about 8 minutes. Add the cheeses and stir until melted, then remove from heat. Stir in the pasta and toss until thoroughly combined. Season with salt and pepper, to taste. Transfer to a serving bowl and serve warm garnished with parsley.