Mum's French onion soup with cheesy baguette toasts

Ingredients

• 25 grams butter
• 4 large brown onions, finely sliced
• 1 teaspoon sugar
• 1/4 cup flour
• 6 cups good beef stock (easy to make your own, see recipe tip)
• 1/2 teaspoon salt and pepper
• 1/2 French baguette
• 2 tablespoons butter, softened
• 1 teaspoon wholegrain mustard
• 1 cup grated gruyère cheese
• small handful of parsley, chopped