Preheat oven to 350 degrees.
Blanch beans 2-3 min in large pot of boiling, salted water until crisp-tender.
Drain; transfer to bowl of ice water. Drain and set aside.
Heat 1 Tbsp of olive oil in large skillet on MEDIUM-HIGH, until oil faintly smokes. Add half of mushrooms; cook, stirring, about 10 min, until dry. Remove from pan. Repeat cooking process with other half of mushrooms. Season to taste with salt and pepper. Let cool.
Mix beans and mushrooms in 13x9-inch baking dish. Stir in gravy; bake about 50 min on center rack of oven.
Top with fried onion pieces; return to oven 10 min.
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