In a small bowl, soak the slivered almonds in the rosewater; set aside.

In a separate small bowl, steep the ground saffron in the 3 tablespoons hot water; set aside.

In a large bowl, combine rice with enough cold water to cover. Using your hand, swirl the rice until the water turns cloudy. Pour off the cloudy water through a fine-mesh strainer, then return the drained rice to the bowl and refill with fresh cold water. Repeat 5 to 6 times until the water runs clear, then drain rice.

In a 5- or 6-quart heavy-bottom pot or Dutch oven, combine the washed rice with 1 tablespoon of the steeped saffron mixture and 7 cups (1.7L) water. Bring to a boil over high heat, then lower heat to maintain a gentle simmer and cook, stirring and scraping the bottom of the pot occasionally, until the rice grains are plump and suspended in a thickened slurry roughly the consistency of regular yogurt, 20 to 25 minutes.

Add sugar and continue to cook over low heat until the sugar is fully dissolved and pudding is glossy, about 10 minutes.

Stir in the remaining 2 tablespoons of steeped saffron, the soaked almonds with the rosewater, and the 3 ½ tablespoons butter until the butter is melted and the saffron has fully integrated into the pudding while stirring occasionally, about 10 minutes. Remove the pot from heat, cover, and let rest for 15 minutes.

Ladle pudding into individual serving bowls or into one large shallow and wide serving bowl and let cool to room temperature, 20 to 30 minutes. Decorate the top with ground cinnamon, pistachios, almonds, and/or rose petals into your preferred decorative pattern only once the pudding has cooled and a thin skin has formed on the surface. Serve.