Grind saffron strands using a mortar and pestle. Sprinkle them over ice cubes in a small bowl and let them melt.
Rinse the basmati rice several times until the water runs clear to remove excess starch.
Place the rice in a pot and add water to cover by 1 1/2 inch. Add in the salt and 1 tablespoon of oil and bring to a boil. Cook for 5 minutes until the rice is slightly tender but not fully cooked.
Break the reshteh polo (Persian noodles) into pieces, add them to the pot with the rice, and cook until the water has almost evaporated.
Drain them in a colander and rinse them quickly with cold water to stop the cooking process.
Mix ⅓ of the reshteh noodles and rice with the bloomed saffron.
In the same pot, add the rest of the oil and sprinkle sesame seeds. Then, place a piece of tortilla at the bottom of the pot for tahdig.
Layer the saffron-infused rice over the tortilla into the pot first, and pack it tight.
Add the remaining rice and reshteh on top and pack it down slightly.
Poke 5 holes into the rice with the handle of a spoon to help steam circulate and dot the surface with 2 tablespoons butter.
Cover the lid with a clean kitchen towel and cook over medium heat for 45 minutes until the rice is fluffy and aromatic and the tahdig is golden crisp.
To make the garnish, heat the oil in a pan over medium heat and cook the onions until golden. Add in the raisins and cook for a couple more minutes.
Once the rice is cooked, turn the heat off and let it sit for 10 minutes. Invert a plate on the pot and carefully flip the rice to reveal the tahdig.
Arrange the caramelized onions and raisins and serve warm.