Melt butter in a very large pot over medium heat. Add onions and garlic; cook and stir until softened and browned, about 20 minutes. Pour in white wine; simmer until reduced by half, 5 to 10 minutes. Stir heavy cream into the saucepan. Bring to a boil; reduce heat and simmer until reduced by half, about 20 minutes. Stir in saffron. Season sauce lightly with salt and pepper. Refrigerate sauce until cool and thick, about 1 hour. Spoon into ice cube trays and freeze until solid.