Heat a dry pan and add the cloves, cinnamon, chillies, black pepper and cardamom, stirring continuously. When they start sputtering, add the coriander seeds. When the coriander starts sputtering, add the cumin seeds. Remove from heat while stirring. You should be able to smell the roasted spices.
Cool down the spices, add the mustard powder and grind to a fine powder in a spice grinder or mortar and pestle.
Cut one of the onions into chunks. In a blender add the onion, ginger, garlic and enough just vinegar to blend into a paste.
In a bowl add the pork, the rest of the vinegar, mustard and 4-5 teaspoons of the roasted spices, salt to taste and the onion-ginger-garlic paste. Mix well. Cover with plastic wrap and refrigerate overnight.
Slice the remaining onion into very fine slices.
In a pan heat the oil and add the bay leaves. Add the finely sliced onions. Fry until they start to turn crispy, then add the pork with its marinade to the pan. Cover, turn the heat to low and simmer until the pork is fork-tender, about 1 hour.