1 (1 3/4-lb) head cauliflower, cut into 3/4-inch-wide florets
1 1/4 lb Yukon Gold potatoes, peeled and cut into 1/2-inch cubes
5 tablespoons vegetable oil
1/2 teaspoon cumin seeds
3/4 teaspoon salt
1 medium onion, finely chopped
2 garlic cloves, finely chopped
2 teaspoons minced fresh jalapeño, including seeds
2 teaspoons minced peeled fresh ginger
1 teaspoon ground cumin
1/2 teaspoon ground coriander
1/4 teaspoon turmeric
1/4 teaspoon cayenne
1/2 cup water

Accompaniment: lemon wedges