Reserve 2 tbsp (30 mL) butter (for topping). Melt remaining butter in large, high-sided skillet set on medium heat. Cook onion and garlic 3 to 5 min. until starting to soften. Sprinkle in flour and ¼ tsp (1 mL) each salt and pepper. Cook, stirring, 2 to 3 min.
Gradually whisk in cream until sauce is smooth; bring to a simmer. Reduce heat to medium-low. Cook, stirring frequently,10 min. until slightly thickened. Remove from heat. Gradually whisk in blue cheese and Parmesan cheese until melted and sauce is smooth. Stir in cooked macaroni. Spread macaroni mixture in 13 X 9-in. (3-L) baking dish.
Melt reserved 2 tbsp (30 mL) butter and toss with breadcrumbs, walnuts and remaining ¼ tsp (1 mL) each salt and pepper. Sprinkle onto macaroni mixture. Broil 2 to 3 min. until golden brown. Garnish with parsley to serve.
Fresh breadcrumbs can be made easily at home by pulsing cubed day-old bread in a food processor until coarsely ground. Breadcrumbs can be frozen for up to 1 month.
Make individual servings by portioning into ten 6-oz (175-mL) ramekins (3-in./8-cm diameter). Arrange ramekins on a baking tray to broil.