Melt ¼ cup of unsalted butter over medium-high heat in a large skillet. Add thinly sliced yellow onions, season with kosher salt and black pepper, and sauté for 25-30 minutes until golden brown.

Add another ½ cup of unsalted butter to the skillet with cooked onions. Stir continuously on medium heat until the butter turns chestnut brown, about 3-5 minutes. Remove from heat.

Bring a large pot of salted water to a boil. Cook linguine according to package instructions until al dente, about 8-10 minutes. Reserve 1 cup of pasta water, then drain and set aside.

In a food processor, combine caramelized onions and browned butter with ½ cup reserved pasta water. Blend until smooth and creamy.

Return the skillet to low heat, add the blended onion mixture, pour in heavy whipping cream and Parmesan cheese. Stir gently until melted and warm.

Add the cooked linguine to the skillet, tossing gently to coat in the creamy sauce. Add reserved pasta water, one tablespoon at a time, for desired consistency.

Serve hot, garnished with more grated Parmesan and chopped parsley. Enjoy every creamy bite!