Directions

• Slice the onion halves, cut-side-down, root to tip. Melt 4 Tbs of the butter in a large, high-sided skillet (see notes) over medium heat, leaving it undisturbed at first, but stirring any unmelted portion once the melted stuff starts to bubble, about 4 minutes. Richly caramelize the onion, with a pinch of salt, in the butter, reducing the heat to low after the first 5 minutes. Stir occasionally (but throroughly) at first, and more frequently as you approach a total of 45 minutes.
• Meanwhile, add reduction paste to 1/2-cup of water and microwave until warm, 30 seconds. Stir to blend. Stir the thyme into the onions, raise the heat to medium, and cook, stirring constantly, 1 minute. Add the broth, stir, then cook until most of the liquid boils away, leaving the mixture wet, but not runny, 4 minutes. Off heat, check seasoning. Spread the mixture out on the bottom and up the side of a large mixing bowl to cool.
• Preheat oven to 375℉ with the rack one slot higher than middle. Toss the baguette slices in 2 Tbs of the oil, a pinch of salt, and a pinch of thyme. Toast them in the oven until thoroughly browned, turning on occasion, 15-20 minutes. Allow to cool.
• Meanwhile, bring remaining 8 cups of water to a boil in the unwashed skillet. Add a generous amount of salt, and cook the pasta according to package directions, 1 minute less than the minimum amount, stirring periodically. Strain. Toss with the onion mixture. Take your time and toss gently but thoroughly as long as it takes to distribute the onions amongst the pasta. Set aside.
• Heat the milk in a medium-sized saucepot over medium heat until hot, but not boiling, 4 minutes. Reduce heat to lowest setting. Don’t proceed until this step is complete.
• Melt the remaining 4 Tbs butter along with the remaining 1 Tbs oil in the unwashed skillet you used for the pasta, over medium heat. Once the butter bubbles, stir until all the butter is melted. (Don’t let it brown.) Add the flour. Cook, stirring, 1 minute.
• Add the milk all at once and whisk thoroughly, vigorously, and continuously, until the mixture becomes thick and starts to bubble in a way that you can’t whisk down, 8 to 12 minutes. Off heat, add mustard. Add pepper, to taste.
• Add all the Gruyère, and all but a handful of the Parmesan, and whisk until it melts. Taste; add salt (perhaps 1¼ tsp) as neccesary. Carefully but thoroughly fold in the pasta mixture a bit at a time. (Yes, it will fit!) Process the toast to crumbs – coarsely if possible, otherwise finely. Top the mac with the crumbs and remaining cheese. Place the pan on the same, unwashed tray that you used for the toasts, and bake until it comes to a lively bubble at the edge, 30-45 minutes. If by that time it’s not as browned as you’d prefer, switch to the broiler for a minute or so. Transfer the dish to a wire rack for 5 minutes; serve.