From here.

• Yellow onions provide a balance of sweet, pungent, savory, and mild onion flavor.
• A small amount of brown sugar helps to accelerate the caramelization process.
• Salting the onions at the beginning of cooking promotes quicker browning, good seasoning, and tender texture.
• Use high heat to kick-start browning of onions, then reduce to medium for gentle cooking.
• Extended sauté time, and reduced heat creates a more complex, sweet onion taste.
• Deglaze the pan with red wine to infuse tasty bits of caramelized onion from the bottom.
• Toasting the bread before adding it to the soup keeps the surface crisper.
• Use a high melting cheese like gruyere for optimal spread when broiled.