Caramelized onions take time but transform burgers with rich sweetness. Slice onions thinly and cook them low and slow in a skillet with a little oil and a pinch of salt. Use medium-low heat and stir occasionally; patience is key—proper caramelization can take 20–40 minutes depending on volume.
To accelerate slightly without burning, add a splash of water, balsamic vinegar, or a teaspoon of sugar midway through cooking. Finish with a pat of butter for glossy flavor and optional herbs like thyme for nuance.
Caramelized onions pair especially well with sharp cheeses and beefy patties, adding depth that complements savory flavors without overpowering the burger.