Generously season roast on all sides with 2 teaspoons salt and 1/2 teaspoon pepper. Let sit for 30 minutes at room temperature before cooking. Heat oven to 400°F.
Combine root vegetables and onions in a large roasting pan or baking dish. One that they can fit in a single layer is good. Drizzle with the 2 tablespoons olive oil, 1 teaspoon salt, 1/4 teaspoon pepper, and rosemary. Toss to combine then spread into an even layer. Nestle the pork roast in the middle.
Transfer pan to oven and roast, until an instant-read thermometer inserted in the thickest part reads 170°F, about 40 minutes-1 hour. Remove from oven and cover with foil to rest while you make the sauce.
For the sauce, melt the 4 tablespoons of butter in a skillet over medium heat. Whisk in the 1 tablespoon of flour until smooth. Slowly add the 3/4 cup marsala while whisking constantly until a smooth sauce forms. Let simmer until slightly thickened. Sauce will continue to thicken as it cools. Whisk in the 2 tablespoons mustard, 1/2 teaspoon salt, and remaining 1/4 teaspoon black pepper.
Place pork roast on a cutting board and slice into thin slices. Serve with vegetables, mustard sauce, and horseradish aioli.