Peel the Yellow Onion (2), then chop them into quarters. Slice the onions so that the pieces form crescent-shaped strips about 3″ long. Chop the Mushroom (4 ounce).
Melt 2 tablespoons of Unsalted Butter (2 tablespoon) in a large frying pan over medium heat. Once butter has melted, add the onions and Salt (1 teaspoon) and toss well. Cook onions over medium for about 30 minutes, stirring often to caramelize all the onions.
When the onions have been cooking for about 10 minutes, bring a large pot of salted water to a boil. Add the Whole Wheat Spaghetti (1 pound) and cook to al dente.
As the spaghetti cooks and the onions are mostly caramelized, move the onions to one side of the frying pan. Add the last tablespoon of Unsalted Butter (1 tablespoon) to the cleared side of the pan. Add the chopped mushrooms to the pan and sauté for 2-3 minutes.
Mix the mushrooms into the onions and add Ground Black Pepper (1/2 teaspoon) and Balsamic Vinegar (1 tablespoon). Add Heavy Cream (1/2 cup) and reduce heat to low, stirring well to incorporate all the cream.
Drain the pasta, reserving about 1 cup of the pasta water. Return it to the pot and pour the onion mixture on top of the pasta. Add 1/2 a cup of the reserved pasta water and mix well until combined. Adjust seasoning to taste.
Plate the spaghetti and add a few crumbles of Gorgonzola Cheese (1/2 cup) as a garnish.