Cut onion lengthwise into 1/4-inch-thick wedges.

Heat 3 tablespoons olive oil in a 10-inch heavy skillet over moderate heat.

Add onion and salt to taste, stirring occasionally, until deep golden and some wedges are crisp, about 15 to 20 minutes.

Stir together curry powder, ground coriander, ground cumin, ground cinnamon, mustard seeds, and cayenne.

Add cashews to onion and cook, stirring occasionally, until nuts are 1 shade darker, about 3 minutes.

Stir in 1 1/2 teaspoons spice mix and cook, stirring, until fragrant, about 30 seconds.

Remove skillet from heat.

Heat remaining 3 tablespoons oil in a 5-quart heavy pot over moderately high heat until hot but not smoking.

Cook remaining spice mix, stirring, until fragrant, about 30 seconds.

Immediately stir in the spinach, mustard greens, dandelion greens, and water.

Cook, stirring occasionally, until most of liquid is evaporated and greens are tender, 3 to 5 minutes.

Season with salt.

Serve greens sprinkled with onion mixture.