Prepare the oven and cups. Heat a conventional oven to 350°F (175°C). Grease all twelve muffin cups thoroughly, including their sides. Set the pan on a stable work surface where it can be filled without carrying an overflowing jug across the kitchen. Dice the onion finely and chop large spinach leaves before heating the skillet.

Soften the vegetables. Heat olive oil in a skillet over medium heat. Cook the onion for about 6–8 minutes, stirring, until softened. Add spinach in batches and cook until wilted and visible liquid has evaporated, about 2–3 minutes. Spread in a clean bowl and let cool briefly while preparing the egg mixture.

Whisk the eggs. Whisk eggs, milk, salt, and pepper until the whites and yolks are evenly combined. Avoid whipping in a large amount of foam, which can make the cups puff unevenly. Use a bowl or jug that is easy to pour from, and whisk again if the mixture has stood while the vegetables cool.

Fill evenly. Divide the cooked spinach-onion mixture and shredded cheddar among the twelve greased cups. Pour egg mixture over the filling, distributing it evenly and leaving a little space at the top. Stir gently within each cup if a mound of cheese or spinach remains exposed. Do not fill one cup to the rim while leaving another half empty.

Bake until cooked through. Bake for about 18–24 minutes, checking the center cups first. The eggs should be set, with no wet center, and reach at least 160°F (71°C). A slight puff is normal. If several cups finish earlier, remove the pan only once the coolest centers are safely cooked rather than judging the browned cheese alone.

Rest and release. Rest in the pan for 5 minutes. Loosen the edges gently with a suitable non-scratching utensil and lift out. Serve two egg muffins per portion, or refrigerate promptly in shallow containers for later. The tops will settle as they cool; do not try to preserve oven-high domes by overbaking the eggs.