Bring a large pot of lightly salted water to a boil over high heat. Add the green beans and return to the boil. Cook until crisp-tender, about 3 minutes. Drain and rinse under cold running water. (The green beans can be prepared up to 1 day ahead. Pat the green beans dry with paper towels, place in a large self-sealing plastic bag, and refrigerate.)
Cut off the stem from each mushroom and slice crosswise into 1/2-inch-thick roundness. Using a teaspoon, scrape out the dark brown gills from the underside of each mushroom cap and cut the caps into 1/4-inch-thick strips.
In a 12-inch nonstick skillet, heat 2 tablespoons of the oil over medium heat. Add the mushroom stems and caps and cook, stirring often, until lightly browned and tender, about 5 minutes. Add the shallots and cook, stirring often, until the shallots soften, about 2 minutes. Transfer to a plate and set aside. (The mushrooms can be prepared up to 2 hours ahead, covered, and kept at room temperature.)
Add the remaining 1 tablespoon oil to the skillet and tilt to coat the pan. Add the bacon and cook over medium heat, turning occasionally, until the bacon is crisp and brown, about 5 minutes. Using a slotted spatula, transfer to paper towels to drain. Leave the bacon drippings in the skillet. When the bacon is cool, coarsely chop and set aside. (The bacon can be prepared up to 2 hours ahead, covered, and kept at room temperature.)
Place the skillet over medium heat and heat until the bacon drippings sizzle. Add the green beans and mushrooms, cover, and cook, stirring occasionally, until heated through, about 5 minutes. Stir in the bacon and season with the salt and pepper. Transfer to a warmed serving dish and serve immediately.