How to Make It
Bring a large pot of water to a boil over high. Prepare an ice bath in a large bowl. Add beans to boiling water, and cook 3 minutes. Transfer beans from boiling water to ice bath; let stand 5 minutes. Drain well.
Stir together milk, pepper, salt, and soup in a slow cooker. Add mushrooms and half of the beans; toss to coat. Add remaining beans; toss to coat. Cover and cook on high until beans are tender, about 3 hours.
Meanwhile, melt butter in a large skillet over medium-high. Add red onion to skillet; cook, stirring often, until just starting to soften, about 3 minutes. Stir in fried onions and cheese. Remove from heat. Place green beans on a serving platter; sprinkle with onion mixture.