Bring 1/2 cup water to boil in small saucepan. Add mushrooms. Remove from heat. Place shallot in small bowl. Add vinegar. Let mixture stand 30 minutes.
Preheat oven to 450°F. Cook favas in large saucepan of boiling salted water until crisp-tender, about 2 minutes. Drain. Rinse under cold water. Drain. Remove and discard outer skin of each fava bean; transfer favas to medium bowl.
Place asparagus on large rimmed baking sheet. Drizzle with 1 tablespoon oil. Sprinkle with salt and pepper. Toss to coat. Roast asparagus until tender when pierced with skewer, about 20 minutes.
Meanwhile, drain mushrooms and slice thinly. Sauté pancetta in saucepan over medium heat until crisp, about 5 minutes. Add remaining 2 tablespoons oil and sliced mushrooms; sauté 2 minutes. Add shallot mixture; simmer 2 minutes. Add favas and toss to heat through. Stir in thyme. Season fava-mushroom mixture to taste with salt and pepper.
Place asparagus on platter. Top with fava-mushroom mixture and serve.