Preheat the oven to 375°F.
To prepare the cauliflower: Toss the sliced cauliflower in a bowl with 1 tablespoon olive oil. Lay slices close together on a parchment-lined baking sheet. Sprinkle with kosher salt to taste and grind some black pepper over top. Bake, flipping halfway through, until the edges are nicely browned and the stems are relatively tender, 25 to 30 minutes. Set aside.
To make breadcrumbs: Feel free to use fresh bread here if you have none that’s stale. Either way, remove crusts, and break into small pieces. (If using fresh, dry it out in a hot oven for a few minutes.) Place bread pieces into a blender along with the garlic cloves, olive oil, and a little salt and pepper. Blend until the contents are uniformly small, coarse crumbs. This should be about ½ cup worth of breadcrumbs. Set aside.
To cook pasta: Bring a pot of salted water to a boil, add pasta, and cook according to package directions until the pasta is just verging on al dente. Drain and set aside.
To make the cheese sauce: In a large pot, melt butter over medium heat, whisking as it bubbles, sputters, and begins to brown, about 2 minutes. Whisk in the flour and continue whisking until flour just starts to turn golden, about another minute or so. Slowly add in milk, whisking all the while to prevent clumps. Add the paprika, about a ½ teaspoon of kosher salt (start with less if using Morton’s or sea salt), and pepper, and be prepared to add more seasoning to taste. Bring to a simmer, continuing to whisk until all ingredients are incorporated. Add the grated cheese, whisking until melted and fully combined. Check the seasoning levels—you may want to add more salt, pepper, or paprika here. Add the cooked pasta and make sure it is fully coated in the cheese sauce.
Increase the oven temperature to 425°F.
Transfer the mac and cheese mixture to an 8 x 8-inch baking dish or a rimmed baking sheet. Arrange the roasted cauliflower slices in a layer on top of the mac and cheese. Add another layer of grated cheese (if you like), and finally, top with an even layer of breadcrumbs. Bake for about 25 minutes. It’s done when the cheese sauce is bubbling and forming a crisp ring around the edge of the dish. The breadcrumbs should be golden brown. Remove from oven and let cool for a few minutes before topping with sliced scallions to serve.