Irish Sausages & Champ

Name: Irish Sausages & Champ
Prep time: 10 min
Total time: 10 min
Cuisine: European, British
Ingredients: Kosher salt; 1/2 c. (1 stick) unsalted butter, melted; 1/2 c. half-and-half; 2 1/2 lb. russet potatoes, peeled, cut into 1 1/2" pieces; 6 scallions, white and light green parts thinly sliced (about 1/2 c.); 2 tbsp. neutral oil, divided; 1 lb. cooked Irish sausages (about 8); 1 large yellow onion, thinly sliced; 1 large clove garlic, finely chopped; 2 tbsp. all-purpose flour; 1/2 c. dark stout beer; 1 1/2 c. low-sodium beef or chicken stock; 1 tsp. Dijon mustard; 1/4 tsp. freshly ground black pepper