Meanwhile, heat a large thick bottomed saucepan or dutch oven with 2 cups oil on medium high heat while you prepare the remaining ingredients: slice ginger, mince garlic, chop the green chilli, measure out the biryani spice mix and set aside. Once the oil is heated, add the sliced onions. Cook, stirring often, until the onions are caramelized and turning golden brown. About 15-20 minutes.
Scoop the browned onions out of the oil and set aside in a bowl. In the same pan with the remaining oil, add 2 tsp mustard seeds, cook until they start to sizzle, about 2 minutes.
Add the sliced ginger, minced garlic, and sliced green chilli. Cook for 2-3 minutes until fragrant.
Add the browned onions back into the saucepan. Reserve about 1-2 tbsp for garnish.
Add the biryani spice mix and 1 cup yogurt. Cook for 5-10 minutes on medium to allow the flavours to combine. You will start to see the oil separate and float over the curry mixture.
Add the boiled potatoes and stir to get them coated in the curry mixture.
Add 2 lbs shrimp and stir to combine. Cook for 3-5 minutes until the shrimp are cooked through and have turned pink. But do not overcook. Give the curry a taste to make sure the salt and spice level it to your liking. Remove the curry from the pan and set aside. (This biryani curry base can be made ahead of time and refrigerated for up to 3 days).