Heat the olive oil in a 26 cm / 10-inch heavy-bottomed pot over medium heat.
Add the onion and scallions. Cook for 5 to 6 minutes, stirring often, until soft and translucent.
Add the garlic and cook for 1 minute, until fragrant.
Stir in the rice and toast for 1 to 2 minutes, coating the grains in oil, until they look slightly glossy.
Pour in the stock and add the salt and pepper. Bring to a gentle simmer, cover, and cook for 10 minutes.
Add half the spinach and stir until it wilts down, then add the rest and stir again until wilted, about 2 to 3 minutes total.
Cover and simmer for 8 to 10 minutes more, until the rice is tender and most of the liquid is absorbed. The mixture should look moist, not soupy or dry.
Remove from the heat and stir in the lemon zest, lemon juice, and dill.
Cover and rest for 5 minutes so the rice finishes absorbing the last of the liquid.
Spoon into bowls and top with crumbled feta, if using.