Preheat oven to 400ºF.
Slice off the top of a head of garlic, just enough to expose the cloves. Drizzle with a teaspoon of the olive oil and wrap in foil. Roast the garlic for 30 to 40 minutes, or until soft and almost caramelized. Let it cool a little so you can handle it, then squeeze out the flesh of each clove out. Mash the cloves with a fork and reserve.
Cook the pasta in a large pot of salted boiling water according to package instructions. Drain, reserving 1/2 cup of pasta water, and reserve back in the pot.
Heat the remaining olive oil in a large skillet, over medium heat. Add half of the minced garlic and sauté until softened and fragrant, about 1 minute. Stir in the chopped Swiss Chard and cook until softened, about 2-3 minutes. Season with salt and pepper, and stir in the lemon juice and zest. Reserve.
Wipe down the skillet with a paper towel and melt the butter over medium heat. Remove and reserve 2 tablespoons for the topping. Add the remaining garlic and cook for a minute, until softened and fragrant. Then, sprinkle the flour and whisk until combined. Cook for a minute or two, until bubbling and starting to turn golden.
Slowly whisk in cold milk. Continue to whisk until it starts to boil. Lower the heat to medium-low and cook, stirring constantly, until slightly thickened, about 8 to 10 minutes.
Stir in the heavy cream, roasted garlic, dry mustard, paprika, freshly grated nutmeg and a pinch of salt and pepper. Cook for another 3 to 5 minutes to thicken.
Remove from heat and gradually stir in the cheese until melted.
In the pot with the pasta, combine the pasta, sauce, Swiss chard and a splash (about 1/4 cup) of the pasta water. If necessary, add more pasta water to make sure the mixture is saucy. Taste and adjust salt and pepper as needed!
Transfer the mac and cheese to a greased 9x13-inch baking dish.
In a small bowl, combine the breadcrumbs, reserved melted butter and garlic powder. Sprinkle over the mac and cheese.
Bake for 15 to 20 minutes or until golden and bubbly.
Garnish with the chopped chives and serve!