• Preheat oven to 350 degrees. Grease a 9x13 deep casserole dish, or a similar sized vessel, with cooking spray. Set aside.

Make the Garlic Mornay Sauce:

• In a medium skillet over medium-low heat, add the chopped bacon and cook until crisp—10-12 minutes. Remove bacon and transfer to a bowl. Remove all but 2 Tbsp of fat from the pan before adding the garlic. Cook over medium-low heat for 2-3 minutes, until golden, then add the chicken bouillon paste and cook for another 1 minute. Add flour and stir into the garlic; cook for 1-2 minutes before adding the warmed milk. Bring garlic béchamel to boil, then reduce heat and simmer until reduced by about one third—2 minutes. Whisk in the heavy cream and bring back to a boil, then reduce heat to low and simmer until thickened and reduced by about a quarter, 2-4 minutes.

How to caramelize the onions:

• Place the onions in large pot over medium-high heat, with butter and a generous pinch of kosher salt. Cook until the butter melts into the onions and steam begins to come off. Reduce heat to low. To ensure and expedite softening, I cover the onions for about 35 minutes with a lid to build steam and keep the moisture in, stirring every 12-15 minutes. Once you remove the lid, cook for 20 minutes, stirring a few times. During the last 20 minutes, add the white wine and allow it to cook off. Continue cooking until the onions have reached your desired level of caramelization.

Make the pasta bake:

• As the sauce reduces, make the pasta—for radiatori, cook in heavily salted water for about 3 1/2 minutes, stirring occasionally, then immediately drain and place into a large mixing bowl—one large enough to fit all of the ingredients. The pasta will continue to cook in the oven, so it should be al dente post boil.
• When the sauce has reduced, remove from the heat and add 1 1/2 cups of mozzarella and 1 cup Gruyere cheese. Stir until smooth; taste for additional salt and pepper, and set aside.
• In a small pot or pan with a lid, add your spinach and 1-2 Tbsp of water: bring heat to medium-low and cover with a lid. Wilt the spinach for 2-4 minutes. Transfer to a cutting board and allow to cool slightly before chopping and squeezing out excess liquid. Add spinach to the large mixing bowl with the pasta.
• Add the 1 cup of caramelized onions, bacon, and chicken to the bowl, then pour in the garlic cheese Mornay sauce and carefully stir until combined.
• Transfer to the baking dish and spread into an even layer. Sprinkle the remaining cheese on top and cover with a piece of parchment and foil. Bake on the center rack of the oven for 20-25 minutes, then remove the foil and return to the oven for an additional 10-15 minutes.
• Finally, leaving the pasta on the center rack, turn the broiler on high. Brown the cheese on top for 2-4 minutes. Remove from the oven, allow to cool for 10 minutes, then serve hot.