Heat oven to 350 degrees F
Coat garlic with olive oil, wrap in foil and roast in hot oven until garlic is soft and golden, 25-30 minutes.
Remove roasted garlic and mash or puree in blender or food processor.
In small bowl, whisk together water and cornstarch until well blended. Set aside.
In a stockpot over medium-high heat, bring to a boil broth, pureed garlic, lemon juice and lemon zest. Reduce heat.
Add cornstarch-water mixture, whisking constantly, until broth thickens and simmers, about 5 minutes.