Add the turmeric, salt, chili powder, and ginger-garlic paste to the beef or lamb. Make sure it is fully coated and let it sit for a least thirty minutes.
In the meantime, tie up all your whole spices into a cheesecloth - cinammon sticks, allspice, black peppercorn, mace, shahzeera, cardamom, cloves.
Add your meat and cheesecloth pouch to the pressure cooker. Cover with about a liter of water or so. Pop on the lid, turn the heat onto high and let it tenderize for about 40 minutes or so (you can leave it in longer or less depending on how tender you want it and how well your pressure cooker works.)
In the meantime, star caramelizing your onions. Turn the heat to medium-high and add in your oil, butte,r and sliced onions to a pan. Allow them to slowly turn dark brown. This will take about 25 minutes or so.
Once your meat is done tenderizing in the pressure cooker, turn the heat off and allow the cooker to cool. DO NOT attempt to open it right away, you will end up burning yourself!
Now take your flour and roast it in a pan on low heat until it turns slightly browner. Make sure you don't burn the flour. Remove immediately and dissolve it in 1/3 cup of water by stirring it until it becomes a slurry. You may also substitute with corn flour (just directly dissolve it in the water). Add that to the nihari and stir.
Keep stirring the nihari until it becomes the consistency you want. If you want it thicker, allow water to evaporate. If you want it looser, add in more water.
Add in the rest of your butter. Taste and adjust for seasonings. Add in a few squeezes of lemon juice.
Serve hot with garnishes!