Red Onion Soup with Balsamic Vinegar and Goats’ Cheese Croûtons

I ate this soup – or one similar – in the Malmaison Hotel in Leith, at an excellent bar lunch. I thought it would be fun and good to experiment with the theme, which is a variation on the Onion Soup with Toasted Cheese in Seasonal Cooking. I do love the mild flavour of red onions, and their caramelizing as they sauté is complemented by the Balsamic vinegar (but you do have to beware not to use too much). I love Balsamic vinegar, used sparingly, and I tend to include a teaspoonful or two in