Melt the butter in a saucepan over a low/medium heat. Grate the ginger, crush the garlic and thinly slice the chili. Add the garlic, chili, onion, cumin seeds, lemon zest and ginger to the saucepan. Stir well to combine and sweat gently for 3 minutes or until tender and fragrant.
Add half the spinach to the saucepan. Cook for a moment until it begins to shrink and wilt. Add the remaining spinach, salt and pepper. Stir to combine until all the spinach has just wilted.
Set aside to cool. Once cool, spoon the yogurt over the spinach. Stir well to combine, coating the spinach in the yogurt. Serve immediately.