Choose a pan wide enough to hold the chicken in a single layer. Heat over medium high heat with the oil in it. Add the mustard seeds and cover the pan while they pop. When they are finished, add the fenugreek seeds and peppercorns. A few seconds later add the curry leaves--stand back, they will splatter! Let the spices cook, stirring, for about a minute.
Add the onions with a pinch of salt and cook until caramelized, about 10-15 minutes. Then add the garlic and cook an additional minute.
Add the spices and the vinegar and stir. Salt and pepper the chicken thighs and add them to the pan in a single layer. Bring to a boil and then cover and reduce the heat to maintain a gentle simmer. Cook the chicken this way for 20 minutes.
Remove the lid and crank the heat up to a brisk boil (high). Add the brown sugar and boil the liquid until it evaporates, leaving behind caramelized chicken (another 10 minutes or so). Taste for salt. Serve with rice, flatbread, or on in its own.
Recipe by The Spiced Life at https://www.thespicedlife.com/vindaloo-inspired-chicken/