Toast spices in a dry skillet on medium-low heat for 3 - 5 minutes, stirring occasionally until mustard seeds begin to pop. Cool completely and then grind into a powder using a spice or coffee grinder.
In a large platter, mix the beef, salt, yogurt, lemon juice, ginger and garlic paste and spice mix. Marinate for 2 hours, or more.
Heat oil in a large non-stick skillet over medium heat. Add chopped onion and fry until caramelized, about 10 minutes.
Add the marinated beef and cook on high heat till the meat is no longer pink, 5-7 minutes.
Remove from stove. Place in Crock-Pot or any slow cooker, add 1 cup water. Cook on low heat for 6 hours or high heat for 3 hours. Serve warm.