okay so according to the Harry Potter Wiki:
Hermione: “Your mother can’t produce food out of thin air, no one can. Food is the first of the five Principal Exceptions to Gamp’s Law of Elemental Transfigura—”
Ron: “Oh, speak English, can’t you?”
Hermione: “It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some…”
So either Mrs Weasley made the sauce in advance, or, much cooler, channeled the ingredients through her wand and made it on the spot. Badass.
Also excuse me if you can increase the quantity why are you letting muggles starve??????