In a large pot, heat the olive oil until shimmering on medium heat. Add the onion, garlic, and mushrooms, and saute on low for 8 minutes. Transfer to a slow cooker.
In the same pot, fill with 4 cups water and bring to a boil. Add the fresh green beans and frozen green beans, and cook for 5 minutes. Beans should be a bright green color and slightly al dente. Drain, and add beans to the slow cooker.
Add the rest of the ingredients to the slow cooker: cream of mushroom soup, almond milk, Worcestershire sauce, onion powder, breadcrumbs, and 1.5 cups of the friend onions (reserve a half cup of the green beans for top of casserole later).
Mix and cook on high for 3-4 hours, until bubbling.
Optional Step - Grease a 13 x 9 casserole dish and transfer green bean mixture into dish. Top with remaining fried onions, and broil for 5 minutes until the top begins to crisp.