Melt butter in a large skillet over medium heat.
Add onions and a pinch of salt and cook, stirring occasionally until tender, about 15-20 minutes.
Add balsamic vinegar and cook for 1-2 minutes longer.
Season chicken breasts with salt, pepper, thyme and garlic powder and add to slow cooker.
Add onions and beef broth. Season broth mixture with additional salt and pepper to taste.
Cover and cook on low for about 5-6 hours.
Scoop out 1/4 cup of broth and add to a small bowl with flour. Whisk together until flour is dissolved. Stir mixture back into slow cooker and cook for another 15 minutes until broth slightly thickens.
Meanwhile preheat oven to 350. *** See below.
Transfer chicken and onion mixture to a baking dish.
Top with shredded cheese and bake in the oven for 10-15 minutes until cheese is melted.
Sprinkle with croutons on top before serving.
***Alternatively you can top with cheese in slow cooker, increase heat to high. Cover and cook until cheese is melted.
Sautéed onions can be prepped 1-2 days ahead of time and kept in a sealed container in the refrigerator until cooking day.