Heat the stock/water in a saucepan. Let it come to a boil and then continue to simmer in low heat throughout the cooking time. You might need less or more water than what's mentioned above. So its better to heat more water and use little by little accordingly.
Place the saffron in a bowl along with 1 tbsp of boiling water and let sit until its infused.
Melt the butter in another heavy pan. Add the onion and the paste. Cook for about 5 minutes until softened.
and cook for 1-2 minutes, until it becomes translucent.
If using wine - add it now along with saffron and cook until the wine has been absorbed. Otherwise, just add saffron and give it a quick stir
until its combined with the rice.
Add 2-1/2 cups of stock to the rice and simmer gently until the rice absorbs the stock. Make sure to keep stirring frequently.
Now add another cup and repeat the process until the rice is all creamy and tender. Make sure to add the stock little at a time since you might not need all the stock.
Once creamy add the zest, salt and pepper. Stir until combined.