Braised eggs with leeks

Name: Braised eggs with leeks
Prep time: 10 min
Cook time: 10 min
Total time: 20 min
Cuisine: European
Ingredients: 30g unsalted butter; 2 tbsp olive oil; 2 large leeks (or 4 smaller), trimmed and cut into ½ cm slices (530g); 1 tsp cumin seeds, toasted and lightly crushed; 2 small preserved lemons, pips discarded, skin and flesh finely chopped (30g); 300ml vegetable stock 200g baby spinach leaves; 6 large eggs; 90g feta, broken into 2cm pieces; 1 tbsp za'atar; salt and black pepper