Preheat the oven to 190°C. An oven-safe skillet — cast iron is the specific recommendation, for its ability to maintain even heat across the base during stovetop cooking and transfer seamlessly to the oven without temperature shock — should be heating on the stovetop simultaneously so it is ready for the vegetables without delay. Melt the 60g of unsalted butter in the skillet over medium heat. Butter is the specific fat choice in this recipe rather than the olive oil of a more traditional shakshuka — butter's milk solids and lower smoke point produce a richer, more rounded, slightly sweet base flavour that works specifically well with the sweet paprika and cumin, and its richness contributes to the finished skillet's satisfying, cosy character. Allow it to foam and settle before adding the vegetables. Add the diced red bell pepper and thinly sliced sweet onion. Cook for 4–5 minutes, stirring occasionally, until both have softened and the onion has turned translucent with slightly golden edges. The onion's natural sugars begin caramelising during this period alongside the butter's milk solids — producing a sweet, slightly browned aromatic base. Add the 4 finely chopped garlic cloves and continue cooking for a further 1 minute until fragrant. Season with kosher salt and black pepper.

Push the softened vegetables to one side of the skillet — creating a small cleared space on the bare pan surface. Add the 1 tsp of cumin, 1 tsp of sweet paprika, and the optional pinch of chili flakes directly onto the exposed hot pan surface rather than onto the vegetables. Leave for 20–30 seconds without stirring. The direct contact between the ground spices and the hot butter-coated surface allows the spices' aromatic volatile compounds to bloom — releasing their essential oils into the surrounding fat at higher intensity than if they were simply stirred into the vegetable mixture. At 20–30 seconds the spices will be visibly fragrant and slightly darkened at their edges; beyond 30 seconds they risk burning. Stir immediately back into the vegetables, folding the spice-infused butter through the entire skillet.

Pour in the 425g of crushed tomatoes and stir to combine everything thoroughly — the tomatoes incorporating the spice-infused butter and the caramelised vegetable base into a unified sauce. Bring to a simmer over medium heat. Simmer uncovered for 5–7 minutes, stirring occasionally. During this time the sauce visibly thickens as its excess water evaporates — reducing to a concentration where the tomato flavour is more intense and the sauce is thick enough to hold an egg in place without the white spreading too widely. The sauce is ready when it has reduced to a consistency that coats the back of a spoon rather than running freely, and the individual tomato pieces have broken down into the sauce. Taste and adjust with additional salt and pepper — the tomato base should be well-seasoned before the eggs are added, as the eggs will draw their seasoning entirely from the sauce beneath and around them.

Reduce the heat to low. Using the back of a large spoon, create 6 shallow wells in the tomato sauce — pressing down gently to form a depression wide enough to contain an egg. Before adding each egg, spoon a small amount of hot sauce into each well — this warms the base of the depression so the egg white begins setting from the bottom as soon as it is cracked in, rather than sitting in cool sauce that delays bottom-setting while the oven heats the top. Crack one egg carefully into each warm well, ensuring the yolk remains intact and centred in its depression.

Transfer the skillet to the preheated 190°C oven immediately. Bake for 8–12 minutes depending on yolk preference — the oven's surrounding heat cooks the eggs from all sides simultaneously, producing the specific gently set white and warm yolk that stovetop-only shakshuka struggles to achieve evenly. Check at 7–8 minutes for runny yolks — the whites should be just set with no visible translucency, and the yolk surface should have a thin, opaque skin from the oven's dry heat while remaining completely liquid when pressed gently. At 10–12 minutes the yolks are softly set throughout — firm enough to break rather than pour when pierced. The eggs continue cooking from residual skillet heat after removal from the oven; always pull 1 minute earlier than the target doneness.

Remove the skillet from the oven. Immediately crumble the 60g of feta cheese evenly across the entire surface — the skillet's residual heat softens the feta slightly, warming it without melting it, so it retains its specific crumbly, slightly briny character. Scatter the fresh cilantro generously over the surface. Bring the skillet directly to the table and serve immediately — shakshuka held for more than 2–3 minutes continues cooking from the cast iron's retained heat and the yolks progress beyond the intended doneness. Serve with bread for dipping alongside.