How to Make It
Melt butter in a Dutch oven over medium; add onion, celery, and garlic, and cook, stirring constantly, until softened, about 5 minutes. Stir in milk, ale, broth, and salt. Bring to a gentle simmer, and whisk in grits. Reduce heat to low; cook, whisking often, until grits are tender and thickened, about 25 minutes. (Liquid should barely bubble.)
Toss together cheese, cornstarch, and cayenne pepper in a bowl. Add cheese mixture to soup, 1/2 cup at a time, whisking until melted after each addition. Stir in mustard and Worcestershire sauce; add salt to taste. Top servings with scallions and Garlicky Pumpernickel Croutons.