GREEK-STYLE RICE WITH SPINACH, FETA, AND BLACK OLIVES

2 pounds fresh spinach, washed and tough stems removed
Coarse sea salt
4 tablespoons extra-virgin olive oil
1/2 cup thinly sliced scallions
3/4 cup finely chopped onions or leeks
1/2 cup fresh tomato purée reduced by boiling to 3 tablespoons
1/2 cup long-grain rice
1/4 cup chopped dill
Freshly ground black pepper
Oily black olives for garnish
Chunks of feta cheese or spoonfuls of thick yogurt for garnish

Wash the spinach and tender stems until the water runs clean; drain. If leaves are large and crinkly, sprinkle lightly with salt and mix well. Let stand in a colander 15 minutes; rinse and squeeze out excess moisture. Shred the spinach to make about 3 cups. Heat 3 tablespoons of the olive oil in a 10-inch straight-sided skillet. Add the scallions, onions, a pinch of salt, and 1/4 cup water and cook, covered, over medium-low heat for 10 minutes. When the water evaporates, slowly let the onions turn golden, stirring occasionally. Add 1 cup water, the reduced tomato purée, and rice. Cover and cook for 10 minutes.