1. MAKE SAUCE Pulse 1 cup canned fried onions in food processor until finely ground; set aside. Melt 3 tbs. butter in large skillet over medium heat. Add mushrooms, ¾ teaspoon salt, and ½ teaspoon pepper and cook until mushrooms release their liquid, about 5 minutes. Increase heat to medium-high and cook until liquid has evaporated, about 5 minutes. Stir in garlic and thyme and cook until fragrant, about 30 seconds. Add flour and ground onions and cook until golden, about 1 minute. Stir in broth and cream and bring to boil. Reduce heat to medium-low and simmer, stirring occasionally, until sauce is very thick and creamy, about 10 minutes.
2. SLOW-COOK Combine sauce and green beans in slow cooker. Cover and cook on low until beans are tender, 5 to 6 hours.
3. TOAST TOPPING Meanwhile, pulse 2 slices bread and 2 tablespoons butter in food processor until coarsely ground. Toast bread crumbs and 2 cups onions in clean skillet over medium-high heat until golden brown, 3 to 5 minutes. Top green beans with bread-crumb mixture. Serve.
MAKE AHEAD: Sauce and topping can be refrigerated in separate airtight containers for 2 days. To finish, microwave sauce, covered, for 1 minute before proceeding with step 2. Crisp bread-crumb mixture in skillet before topping beans.
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