Thinly slice onions and fry them in oil till golden and crisp. Set these aside half of this.
In the same pan, add spices and saute till they begin to sizzle.

Add ginger garlic paste and fry till the raw smell goes off.
Add parboiled potatoes and the rest of vegetables. Fry for 2 to 3 minutes.

Add turmeric, masala powder and red chilli powder. Mix and fry well for 2 minutes till you begin to get an aroma.
Pour yogurt and mix. Cook till the vegetables are done to tender. You can cover and cook. Cook uncovered if there is lot of moisture after the veggies are cooked.

While the veggies cook. Bring lot of water to a boil and cook rice till al dente along with bay leaf and shahi jeera. The rice has to be cooked grainy and not mushy. Drain it off in a colander. Soak saffron in 3 tbsps of this hot water or hot milk. Set aside.
If you like to make a 2 layer veg biryani, set aside 1/3 of the cooked vegetable mix. Now mix together the rest of the cooked veggies, rest of the yogurt, mint, coriander, nuts and raisins, few fried onions.

Layer the rice over the vegetable mix. Then layer the 1/3 cooked vegetable mix or 2 pinches of masala powder. Sprinkle some fried onions, mint and coriander.
Layer rice over it again, then the fried onions, mint and coriander leaves and pour off the saffron milk. Fry nuts in a tbsp ghee, saute raisins and then pour this over the rice. Place a moist clean cloth to seal the rim. You can even use a foil or atta to seal the utensil.

Place the dum utensil on a hot tawa and cook for 5 minutes on a medium flame and then on low for 5 to 7 minutes based on the thickness of your utensil.