As Anthony Bourdain once famously said "onion soup, unsurprisingly, is all about the onions." What is surprising though, is that most recipes for onion soup fail to mention which onions to buy. You want to use onions both high in acidity and high in sweetness, to create a perfectly flavored soup. Both Vidalia onions and yellow sweet onions will work.
Most important of all, of course, are those glorious cheese toasts. Make sure to toast your bread before melting cheese over it. This will prevent your toast from getting too soggy too fast. We love rubbing some garlic over the toast before melting the cheese.
And remember, your soup can never be too cheesy!
1/3 cup of butter, plus a little extra for the toasts
4 onions, peeled and thinly sliced
1 tbsp plain flour
3 sprigs thyme, leaves picked
1 tbsp balsamic vinegar
1.75 cup of cider, white wine, port, or cider syrup
2.5 cup of good-quality beef stock
8 slices of baguette
1 clove of garlic, halved
4 ounces of Gruyère, grated
Melt the butter in a large, heavy bottomed pan over a low heat. Add the onions, season and cook, stirring regularly, until caramelised and deep brown. Once they've softened, you can turn up the heat a little, but keep an eye on them. This will probably take between 90 minutes to 2 hours.
Stir in the flour and thyme and cook for a couple of minutes, stirring, then add the vinegar and a third of the cider, stirring and scraping all the brown bits from the bottom of the pan. Whisk in the rest of the cider and the stock, and bring to the boil. Simmer for about an hour. Meanwhile, heat the grill and rub the baguette slices with the cut side of the garlic clove. Brush with melted butter, and toast on both sides.
Taste if the seasoning is to your liking. To serve, ladle into ovenproof dishes and top with 2 slices of toast and heaping spoons of cheese. Grill until golden, then serve immediately.