Trim the green beans.
In the slow cooker insert, whisk the cream of mushroom soup, minced garlic, butter, flour and heavy cream. Add a few pinches of salt and pepper. Gently toss in the green beans, until covered in the sauce.
Set on High and cook for 2 hours.
Sprinkle fried onions on top and cook another 20 minutes.
If you want the fried onions really crispy, transfer the green beans and sauce to baking dish and run under the broiler for 1 minute.
Serve Slow Cooker Green Bean Casserole as side dish for holiday meal or weeknight dinner.