In a large skillet, add a drizzle of olive oil, the onions, and a sprinkle of salt. Turn heat to medium-low and cook slowly for 30 minutes to 1 hour until very soft and golden, stirring every few minutes.

Meanwhile, make the rest of the mac. In a pot (I use the pot from boiling the pasta), add the butter, garlic, rosemary, bouillon, if using, and dijon over medium-high heat. Stir to melt and create a paste. Sprinkle the flour over the mixture and stir for one minute. Pour the milk into the flour mixture slowly, stirring or whisking as you pour. When all the milk is added and the mixture has bubbled, stir in all the cheese to melt. Taste the cheese sauce and add salt to your preference.

When the cheese sauce and the onions are done, add the cooked pasta, onions, and most of the walnuts to the sauce. Stir together and taste one more time for seasoning. Pour the mac into a baking dish and top with the rest of the walnuts and the panko bread crumbs.

Bake at 375F for 20-30 minutes until the cheese sauce is bubbly and the top is lightly browned. Eat. Enjoy!