The meat doesn't have to be chicken, I use lamb and pork regularly.
Start by peeling and chopping 3 medium onions, red is more authentic due to the sweetness they impart, but white onions work well. Take a large pan and add enough vegetable oil to cover the bottom of the pan, place on a medium high heat and add your chopped onions, along with a pinch of salt and start let them brown stirring occasionally.

2 pieces of ginger root, peeled
4 green chillies
6 cloves of garlic
If you don't have time to use a pestle and mortar, place into a your favourite food processor and add the following spices.
2 teaspoons garam masala
1 teaspoon ground coriander
1/2 teaspoon ground cumin
1/2 teaspoon turmeric
1/2 teaspoon ground cinnamon
1/2 teaspoon cayenne pepper

In addition to the spices add 1 teaspoon of salt and the juice from a tin of tomatoes and then blitz into a paste.
When the onions have browned, add your masala paste to the pan, turn the heat to high and start to fry the paste for a couple of minutes.
Then add 4 chicken thighs,skin removed, but left on the bone.
Leaving the bone in the thigh adds flavour and can be easily removed after cooking. Chicken legs can be used but it's more difficult to remove the skin.
Turn chicken thighs so they get evenly coated in the masala, then add the tomatoes from the tin used to make the masala paste along with a splash of malt vinegar. Stir well and then add enough chicken or vegetable stock to almost cover. If you don't have stock, use good old fashion H2O.

Serve with rice.