In a large skillet set over medium-high heat, melt ¼ cup of unsalted butter. Add sliced yellow onions along with ½ teaspoon of kosher salt and ¼ teaspoon of black pepper. Sauté for about 25–30 minutes until the onions turn soft, deep golden brown, and caramelized.

In the same skillet, melt ½ cup of unsalted butter over medium heat. Stirring constantly, watch carefully for the butter to turn a beautiful chestnut brown and develop a nutty aroma, about 5–7 minutes.

Bring a large pot of salted water to a rolling boil. Add 8 ounces of linguine and cook according to package instructions until al dente, typically around 8–10 minutes. Before draining, reserve 1 cup of the starchy pasta water.

In a food processor, combine the caramelized onions, the browned butter, and ½ cup of the reserved pasta water. Blend until smooth. Return to the skillet, adding 1 cup of heavy cream and 1 cup of grated parmesan cheese.

Gently toss the drained linguine into the skillet with the sauce, stirring well to ensure the pasta is evenly coated.