3 cups canned fried onions
2 slices high-quality white sandwich bread, torn into quarters
5 tablespoons unsalted butter
10 ounces white or cremini mushrooms, trimmed and sliced ¼ inch thick
4 garlic cloves, minced
2 teaspoons minced fresh thyme or ½ teaspoon dried
¾ teaspoon salt
½ teaspoon pepper
¼ cup all-purpose flour
1¼ cups low-sodium chicken broth
1¼ cups heavy cream
2 pounds green beans, trimmed and cut into 1-inch lengths
1. Pulse 1 cup fried onions in food processor until finely ground, about 10 pulses; set aside. Pulse bread in food processor to coarse crumbs, about 10 pulses. Melt 2 tablespoons butter in 12-inch skillet over medium heat. Add bread crumbs and remaining 2 cups fried onions and toast, stirring often, until golden brown, 5 to 7 minutes; set aside for serving.
2. Wipe skillet clean with paper towels. Melt remaining 3 tablespoons butter in skillet over medium-high heat. Add mushrooms, garlic, thyme, salt, and pepper and cook until mushrooms are softened and lightly browned, 8 to 10 minutes.
3. Stir in processed fried onions and flour and cook for 1 minute. Slowly whisk in broth and cream, scraping up any browned bits and smoothing out any lumps. Bring to simmer and cook, stirring occasionally, until very thick and creamy, about 10 minutes; transfer to slow cooker.
4. Add beans to slow cooker and coat evenly with sauce. Cover and cook until beans are tender, 4 to 6 hours on low. Sprinkle with reserved bread-crumb mixture before serving. (This dish does not hold well on warm setting.)