Season the beef chuck roast liberally with salt and pepper. Sear in a pan over medium-high heat for 4-5 minutes on each side until a rich crust forms.

Transfer the seared roast to a slow cooker, pour in the beef broth, cover, and cook on low for 8 hours or until fork-tender.

Thinly slice yellow onions. Melt butter in a skillet over medium heat, add onions, and cook for 30-40 minutes until golden brown.

Mix mayonnaise, minced garlic, and a pinch of salt in a bowl. Optional: add lemon juice to brighten flavor. Set aside.

Slice the rested roast thinly against the grain. Layer beef and caramelized onions onto halved crusty rolls.

Ladle warm broth into small bowls for dipping. Drizzle garlic aioli over sandwiches before serving.