Special equipment: a food processor fitted with the slicing blade attachment (optional)
For the soup: Place a large pan over medium heat with a drizzle of oil and the butter. Peel and very finely slice the onions. This is a big task, but using a food processor fitted with the slicing blade attachment should make things a bit easier.
Add the onions to the pan with the bay leaf, pop the lid on and leave to cook until soft, about 25 minutes. Give the onions a good stir every now and then so they don¿t burn. If they look like they are catching at any time, just add a little more oil.
Meanwhile, peel and finely chop the garlic, then finely chop the sage leaves and set both aside.
For the croutons: Preheat the oven to 300 degrees F. Trim the baguette ends and cut the baguette into eight thick slices (about 1-inch thick) and lay them out on a baking sheet.
Next, roughly grate the Gruyere cheese (or Parmesan), sprinkle the mustard powder over and toss it all about to mix together. Then spread it evenly over the tops of the bread slices.
Check on the onions, giving them a good stir every now and then.
Once the onions are a few minutes off being ready, place the croutons in the oven to bake for about 4 to 5 minutes. You could also grill them for about 5 minutes of you prefer.
Once the onions are lovely and soft, add the garlic, sage and flour, giving them a good stir in. Pour in the beef stock, put the lid back on, increase the heat and bring the soup up to the boil. Then leave it to bubble away for 2 to 3 minutes before removing it from the heat. Season to taste with salt and pepper.
Remove the crisp, melted cheese-topped croutons from the oven. Pick and roughly chop the parsley leaves.
Divide the soup among 4 wide serving bowls and serve with the croutons either sitting right on top of the soup or to the side of the bowl. Scatter the parsley over and serve.
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