Reduce heat to medium and melt butter the same skillet (or a separate, small saucepan if desired). Saute garlic for 30 seconds, until fragrant. Add flour and whisk until evenly combined and cook for 1 minute. Slowly whisk in stock, then slowly whisk in milk and season with pepper. Cook until mixture reaches a simmer and thickens. Remove from heat and stir in cheese until melted.