- Preheat oven to 200ºC (400ºF). Heat oil in large skillet set over medium heat. Cook shallots 3 to 5 min. until starting to brown. Add mushrooms, garlic, thyme, mustard, salt and pepper. Cook 6 to 8 min. until mushrooms start to brown. Sprinkle on flour and stir through. Cook 2 min. until no dry flour remains. Remove from heat.
- Stir in sour cream until sauce is smooth. Fold in green beans and Parmesan. Spoon into greased 13 x 9-in. (3-L) baking dish. Sprinkle fried onions overtop.
- Bake 25 to 35 min., until golden brown and bubbling. Let stand 10 min. before serving.
This dish can be assembled, covered and refrigerated up to 8 hr. before baking.