Rine & soak Basmati rice in room temperature water for 1 hour. Sauté the onions and mushrooms in oil until golden brown (around 5-7 minutes).
Add in the spices and aromatics; sauté for another 2-3 minutes. Add in the peas.
Add 4 cups of water and bring everything to a simmer. Turn down the heat and simmer COVERED for about 15 minutes or until the rice is al dente. Turn off the heat and leave to steam for another 7-10 minutes.
In the meantime, gently heat the milk until it barely simmers. Turn the heat off and add the saffron threads to steep for at least 10 minutes.
Garnish the cooked Biryani with the saffron infusion and knobs of butter just before serving; enjoy immediately.