Melt 2 tbsp butter in a 12-in. (30-cm) skillet set over medium-high heat. Saute mushrooms with mirepoix mix, garlic and rosemary 10 min. or until vegetables start to soften.
Sprinkle flour over top. Cook, stirring 2 min. Gradually stir in broth. Bring to a boil. Stir in chicken. Reduce heat to low. Simmer 20 min. or until sauce has thickened and vegetables are tender.
To make crumble topping, melt remaining 2 tbsp butter and combine with oats in medium bowl. Stir in cheese. Sprinkle on top of chicken mixture in skillet. Place under broiler for 3 to 4 min., or until topping is golden.
Add cooked peas or green beans to pie filling. For richer topping, mix 2 tbsp (30 mL) cooked, crumbled bacon to oats mixture.
*Mirepoix mix is equal parts diced carrot, celery and onion.