Preheat oven to 325 degrees F.

Season the chuck roast with a generous amount of salt and pepper. Place in a roasting pan and add sliced red onions, 4 smashed garlic cloves, ½ cup balsamic vinegar, 1 ½ cups beef stock, and 1 bay leaf. Cover and braise in the oven for 2 ½ - 3 hours or until the beef shreds easily. Shred the beef and keep warm.

Meanwhile, pickle the red onions. Combine water with vinegar, sugar, and salt. Add the red onions and let sit for at least 30 minutes.

Prepare the horseradish cream by combining creamed horseradish with mayo. Set aside.

Prepare the waffles. Combine 1 ⅓ cup all purpose flour with 4 teaspoon baking powder, ½ teaspoon salt, 2 teaspoon sugar, 2 egg yolks, ½ cup melted butter, and 1 ¾ cup milk. Whisk the mixture until combined.

Beat the egg whites until moderately stiff. Fold in the whites to the waffle batter just until incorporated. Heat your waffle maker and cook the waffles until golden brown. Toast the waffles if desired for an extra crispy waffle.

Assemble the sandwiches. Top half of the waffle with a generous portion of the braised beef. Spread a dollop of the horseradish cream on top, followed by the red onions and arugula. Top with the other half of the waffle. Serve immediately.