Mix water, lemon juice, 2 cups of sugar, and the crushed cardamom and saffron in a saucepan. Cook on low heat until the sugar is dissolved, the let it let it simmer for at least 30 minutes.
Meanwhile, peel and boil the sweet potatoes until they are very soft, for about 15-20 minutes.
Once cooked, drain the potatoes and transfer to a bowl.
Mash the sweet potatoes using a fork or electric mixer, until they are smooth.
Mix the mashed sweet potatoes with flour, lemon zest and ¼ cup of sugar and set mixture aside to cool.
Once cooled, form 1-inch balls by rolling the sweet potato mixture between your palms.
Remove the saucepan of sugar water mix from heat and let it cool.
Heat the oil in a pot or wok, and test the temperature with one of the Gulab Jamun balls: if it turns dark in less than a minute, reduce the heat.
Fry the Gulab Jamun balls for 10 minutes (they should turn light brown) and then put them quickly into the syrup.
Let the balls soak in syrup for 10-20 minutes
Serve topped with nuts (optional) and additional syrup.