To make the cashew cream, in a medium bowl, add cashews and cover with boiling water. Let soak for 30 minutes. Drain well.
In a high-speed blender, add drained cashews and ½ cup water. Blend until smooth, adding more water as needed to get a creamy consistency (I added an additional ½ cup water). Season to taste with salt (I like ¼ teaspoon).
In a large skillet or Dutch oven, melt butter over medium-high heat until foaming subsides. Add mushrooms, garlic, 3/4 teaspoon salt, and 1/8 teaspoon pepper. Cook until the mushrooms have released most of their liquid, about 5 to 7 minutes.
Stir in flour and cook for one minute. Whisk in broth, scraping up any brown bits from the bottom of the pan. Bring to a boil, whisking constantly.
Reduce heat to medium. Whisk in cashew cream and simmer until the sauce has thickened, about 5 minutes. Season to taste with salt and pepper.
Meanwhile, cook the beans in the microwave according to the package instructions (or microwave on HIGH power for 4 to 5 minutes; let stand 1 minute before removing from the microwave). Drain well (I like to use a salad spinner to spin them dry).
Add the green beans to the pot with the mushroom sauce and toss until evenly coated. Transfer the beans and sauce to a crock pot.
Cook on HIGH for 2 hours or LOW for 4 hours or until a thermometer inserted in to the middle of the casserole reaches 165 degrees. Top with canned fried onions immediately before serving.