Heat a drizzle of olive oil in a large non-stick pan over medium heat. Add the leek and cook for 5–6 minutes, or until softened.
Stir in the rice and white wine, cooking for 1–2 minutes until the wine has mostly absorbed.
Gradually add the stock, one ladle at a time, stirring frequently and allowing the rice to absorb the liquid before adding more. Continue until the rice is tender and creamy, about 18–20 minutes.
Add the spinach and cook for 1 minute, just until wilted. Remove the pan from the heat and stir through the butter and pecorino. Season with salt and pepper to taste.
Serve immediately, topped with crumbled feta and a little lemon zest if desired.