you should always cook onions right down, at least until golden (20 mins minimum). Undercooked onions are a cardinal sin in any form of cooking!
@jam Not all curries cook onions till brown but quite a few do. It forms the base thickness of the sauce. Some tips: absolutely make sure you fry off any dry spices before adding any liquid. But burning them is a irreversible mistake (start again if you do). Burning garlic is equally as bad. Yogurt in curries can split and look like curdelled milk which is edible but an epic fail. Use only full fat yogurt and add in stages to the pan with no liquid (after the dry spices go in). Sizzle it spoon by spoon and mix it in slowly and it should not split.
Marnanating meat in spices and yogurt is a really good technique but normally needs to be over night. A tikka whole lamb leg is spectacular when done like this and then cooked over coal. The yogurt is meant to tenderise the meat but not sure how true that is.
Anyway besides all this the very best slow cooked beef curry IMO is Malaysian beef Rendang and whilst having lots of ingredients is pretty fool proof. You don’t do any spice frying or owt like that. Put that in the slow cooker all day and I promise you it will be one of the best curries you ever tasted. Will whatzapp you the recipe.