Preheat the oven to 430°F (220°C). Rub the brisket on all sides with salt, pepper, mustard seeds, rosemary, thyme, and smoked paprika. Drizzle with olive oil and wrap tightly in parchment paper, then double-wrap in aluminum foil. Place in a roasting pan and bake for 1 hour. Lower the temperature to 250°F (120°C) and continue roasting for 5 to 6 hours, until very tender.
Remove the brisket and let it cool slightly. Transfer to a wire rack set over a sheet pan. Reserve 2 tablespoons of the juices and solids from the roasting pan for the glaze.
To make the glaze, whisk together the reserved juices, mayonnaise, Dijon, barbecue sauce, garlic, whiskey, and smoked paprika until smooth. Brush the glaze generously over the top of the mustard brisket (no need to flip). Return the meat to the oven, uncovered, at 350°F (180°C) for 45 minutes.
Place the potatoes, rosemary, garlic, and salt in a large pot and cover with water. Bring to a boil, reduce heat, and simmer until very tender, about 45 minutes. Drain, reserving some of the cooking liquid. Mash the potatoes, then mix in olive oil, white pepper, mustard, and enough reserved water to reach a smooth, creamy consistency. Taste and adjust salt.
For the sauce, cook the onion in a skillet over medium heat with a pinch of salt until deeply golden, about 15 minutes. Add the red pepper flakes and maple syrup and cook for 3 minutes longer. Pour in the wine and simmer over low heat until slightly reduced and syrupy, about 20 minutes. Season with salt and pepper to taste.
To serve, spoon generous scoops of mashed potatoes onto each plate, top with 2 to 3 thick slices of mustard brisket, and finish with a spoonful of the warm onion-maple sauce.