Melt 2 tbsp (30 mL) butter in 12 in. (30 cm) skillet set over medium-high heat. Stir in mushrooms, carrot, celery, onion, garlic and rosemary; cook 5 min., or until vegetables start to soften.
Sprinkle flour overtop. Cook, stirring, 2 min. Gradually stir in broth, ensuring sauce is smooth. Bring to a boil. Stir in chicken. Reduce heat to low. Simmer about 20 min., or until sauce has thickened.
To make crumble topping, melt remaining 2 tbsp (30 mL) butter and combine with oats in medium bowl. Stir in cheese. Sprinkle on top of chicken mixture in skillet. Place under broiler 3 to 4 min., until topping is golden.
Cooked peas or green beans can also be added to filling.
For a richer topping, mix 2 tbsp (30 mL) cooked, crumbled bacon into oats mixture.