French Onion Soup with Mini Gruyère Toasties (aka mini grilled cheese!)

white onions (, sliced very thinly (with a knife/mandoline/food processor))

salted butter

garlic cloves (, minced)

red wine

hot, good-quality beef stock

brandy

balsamic vinegar

salt and freshly ground black pepper (, to taste)

slices of white bread ((see notes))

knob of butter (, for spreading + frying)

Gruyère cheese, thickly sliced