As you progress through the game you'll get the quest called "Farmer's Market" from the hippy looking girl. Completing the quest will allow you unlock the ability to purchase an expansion. The expansion is to the left of where your restaurant started, and you'll see a couple of stalls (Broccoli and Romaine Lettuce), and a pile of wrapped boxes.
Garlic is used in a number of dishes including; Garlic Toast, Ranch Dressing, and Steak Florentine
You can also harvest at your neighbors stall for a chance to collect more garlic.
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