Cut cauliflower florets and stem into 1-inch pieces.
Pour water into a medium saucepan to a depth of 1/4"; add cauliflower and garlic. Cover, bring to a boil over high heat, and cook until florets are very easily pierced with a paring knife, about 10 minutes. Drain and set aside.
Meanwhile, heat 2 tablespoons oil in a large skillet over medium. Add onion, cumin, and 1 teaspoon salt. Cook, stirring often, until onion is translucent, about 3 minutes.
Increase heat to medium-high and cook, stirring often, until onion is golden brown, about 5 minutes more.
Transfer onion mixture to a blender. Add yogurt, lemon zest and juicecauliflower, and remaining 1 teaspoon salt and purée until smooth.
With the motor running, add 2 tablespoons oil in a steady stream until well combined. If dip is too thick, blend in 1 tablespoon water at a time to reach desired consistency.
Let cool to room temperature.
Makes about 4 cups
Dip can be made 5 days ahead; cover and chill.