Place cut green beans in a pot and cover with water. Bring to boil and cook until tender, about 7 to 10 minutes. Drain, place in slow cooker.
In a medium mixing bowl, whisk together milk, salt, pepper and cornstarch. Set aside.
In a medium saucepan, heat up oil. Add onion and garlic and saute for 2 minutes, stirring often. Add sliced mushrooms and cook until fragrant and soft.
Add milk and cornstarch mixture, ½ cup at a time. Cook until thickens. Stir often.
Pour mixture over beans. Gently stir to coat.
Place lid on slow cooker, set to LOW setting and cook for 2.5 hours.
When done, set to warm until ready to serve.
Preheat oven to 375 degrees F. Arrange French fried onions in a single layer on baking sheet. Toast up for 2 to 3 minutes ( watch them so they won't burn) . Sprinkle over cooked casserole.
Notes
To make ahead: make sauce, cool completely, cover and refrigerate for up to 2 days. Cook green beans, drain, cool completely, cover refrigerate up to 1 day.
Please note, that the nutrition value can vary depending on what product you use. The information below is an estimate.