Bangers and Mash with Guinness-Onion Gravy

Ingredients

• 2 pounds Yukon gold potatoes, peeled and cut into 1-inch pieces
• ½ teaspoon table salt, plus salt for cooking potatoes
• ¾ cup half-and-half, warmed
• 10 tablespoons unsalted butter, divided
• 2 pounds bratwurst
• 1 onion, halved and sliced thin
• 1½ tablespoons all-purpose flour
• ¾ cup beef broth
• ¾ cup Guinness Draught beer
• 1 tablespoon Dijon mustard
• ½ teaspoon pepper