First, you take the butter and put it in a large pot over medium flame. Add the chopped onions in it and stir so that the onions are properly caramelized by butter. Once you have covered the onions in butter properly, add garlic, thyme, bay leaves, and salt and pepper to the pot and let it all cook till the onions become translucent or slightly brownish. This would take you around 15 to 20 minutes. 

Next, you add wine to your onions and bring it to a boil, after which put it on low flame till you see the wine evaporate and onions dry.

Now, carefully remove the bay leaves and thyme sprigs from the pot and dust the onions with some flour. Make sure that it is on low flame so that the flour doesn't start to burn. You can also keep stirring the whole mix for the raw flour to cook well without getting burnt.

Finally, add beef broth and let the soup simmer for 10 minutes. Season it with salt and pepper as per your taste.

All you need to do now is preheat the broiler when you're ready to eat it. Lay down the baguette slices on a baking sheet and cover them with the Gruyere cheese. Let it broil for 3 to 5 minutes until it turns bubbly and golden brown.

Your soup is all set to be eaten now - just serve it in bowls and top it off with several or as many pieces of Gruyere croutons as you want. Alternatively, you can also serve the French onion soup with 2 slices of bread layered with cheese, melted and toasted in an oven. So what are you waiting for? Go get started with this yummy goodness!