In a saucepan, add 2 tablespoons butter and heat it up on slow heat. As the butter starts to melt and forms a bubbly texture, add 1 teaspoon of fresh garlic paste. Sauté for a few seconds and add chopped onion. Give a good stir and cook the onions until become translucent.
Add 1 tablespoon Cajun seasoning and mix it well. Deglaze with pasta water or broth. Mix it well on slow heat and add heavy cream. Quickly stir the sauce for a few minutes to blend all the ingredients.
Now add 1 tablespoon butter to add a shiny smooth texture of sauce. Simmer the sauce for a minute on low heat. Add freshly grated parmesan cheese and mix it well.
Add freshly chopped parsley and again give it a good mix.
Monitor the sauce consistency. If it's runny cook it to get thick.
Blend the sauce with hand blender and serve.