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.