- Preheat oven to 175°C (or 155ºC for fan forced oven). Thinly slice the red onion and dice the tomato with no seeds.
- In a medium saucepan, melt butter over medium heat until foaming. Add red onion and mixed herbs, and cook. Stir frequently until it begins to darken and brown, for about 5 minutes total. Add the tomato. Reserve.
- Grease 2 ramekins or small bowls (150ml) with butter. Lightly sprinkle salt and pepper on the bottom and side of each ramekin. Scoop enough onion and tomato mixture to form a thin, even layer on the bottom of each ramekin.
- Form a circular depression in the centre of the mixture in each ramekin. This will help to keep the egg yolk centered.
- Gently slide an egg into each ramekin. Drizzle 1 teaspoon of milk around each egg, then season with salt. Top each egg with a light layer of grated cheddar cheese.
- Place ramekins in a baking dish and transfer to oven. Carefully pour enough boiling water into baking dish to submerge ramekins 3⁄4 of the way. Bake eggs in cocotte until whites are just set and yolks are still jiggly – for about 15 minutes. Very carefully remove baking dish from oven.
- Using a very thin metal spatula and tongs, lift each ramekin out of the hot water. Set ramekins on a clean kitchen towel to dry the bottom, then transfer to plates. Serve with avocado and toast on the side.