Heat the oven to 425°F. Put the sweet potatoes in a medium saucepan and add enough cold water to cover them by about 1 inch.

Set the saucepan over high heat and bring the water to a boil.

Lower the heat to medium and cook the sweet potatoes for 12 to 15 minutes, until a fork slides in easily.

Drain the sweet potatoes in a colander and return them to the empty saucepan.

Add 0.33 cup of the coconut milk to the saucepan. Mash the sweet potatoes until mostly smooth, season with salt and pepper, and set the saucepan aside.

Heat the olive oil in a large skillet over medium heat.

Add the onion to the skillet. Cook for 6 to 8 minutes, stirring now and then, until the onion is soft and lightly golden.

Add the ginger and garlic to the skillet. Cook for 30 seconds, stirring, just until fragrant.

Stir the curry powder and tomato paste into the skillet. Cook for 1 minute, stirring, until the tomato paste looks a shade darker and coats the onion.

Add the chickpeas to the skillet. Stir for 1 minute so they pick up the spiced onion mixture.

Use the back of a spoon to crush about 1 cup of the chickpeas right in the skillet. This helps the filling hold together under the topping.

Add the diced tomatoes, the remaining coconut milk, and 0.5 cup water to the skillet. Bring the mixture to a gentle simmer.