• Preheat the oven to 350-degrees Fahrenheit (175℃)

To make the pasta:

• Cook the pasta just shy of the package directions. Drain. Stir in 1 tablespoon of olive oil, set aside, and keep warm.

To make the caramelized onions:

• In a large heavy bottom skillet over medium-high heat, add the avocado oil and the sliced onions. Stir well to coat the onions in the oil. Allow the onions to cook without stirring for approximately 5 minutes.
• Reduce the heat to medium. Add the salt, pepper, and thyme to the onions and stir well. Continue to cook the onions, stirring only occasionally, until the onions are deeply caramelized and golden brown (approximately 20-25 minutes). 
• Remove the onions to a bowl and discard the thyme sprigs. Set the onions aside.

To make the cheese sauce:

• Melt the oil and butter in the same large skillet, over medium heat. Add the flour and cook, stirring occasionally for 1-2 minutes.
• Add half of the milk to the butter-flour mixture (roux) and stir well to combine. Add the remaining milk to the skillet and stir well until the mixture thickens.
• Season the thickened sauce with a pinch of salt, a pinch of pepper. Add the cubed and shredded cheddar, jack, and mozzarella cheese to the skillet, and stir continually until the cheese is completely melted and creamy.
• Add the apple cider vinegar to the cheese sauce along with the caramelized onions, and the cooked and drained pasta. Stir well to combine.
• Spoon the French onion macaroni and cheese evenly into a 4-quart casserole dish that has been prepared with cooking spray.
• Bake the casserole in a preheated oven, uncovered, for 30 minutes or until the cheese lightly golden and bubbly.