Preheat the oven to 200°C (400°F).

Bring the water and coarse salt to a boil in the large pot.

Cook the basmati rice in the boiling salted water until very al dente, about 8 to 10 minutes.

Drain the rice in the colander.

Rinse the drained rice under cold water to stop the cooking.

Grind the saffron with a pinch of coarse salt using the mortar and pestle.

Add a few drops of boiling water to the ground saffron and let it bloom for 2 minutes.

Whisk the plain yogurt, egg yolks, and bloomed saffron in the mixing bowl.

Add the cooled basmati rice to the saffron mixture.

Fold the rice until every grain is evenly coated with the saffron mixture.

Generously coat the oven-safe baking dish with the olive oil and butter.

Scatter the spring onions and dried cranberries over the base of the baking dish.

Pack the saffron-coated rice firmly into the baking dish.

Cover the baking dish tightly with aluminum foil.

Bake the tahchin for 80 minutes at 200°C (400°F).

Unmold the tahchin onto a serving plate and serve while the rice crust is crisp.