Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a small bowl, whisk together the flour, baking soda, salt, and nutmeg to evenly distribute the dry ingredients.
Roughly chop 1 cup of Cadbury mini eggs into 1/4-inch pieces for the dough and crush 1/4 cup into smaller bits for topping.
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar on medium speed for 2-3 minutes until light and fluffy.
Add the room-temperature egg and vanilla extract to the creamed mixture and mix until smooth and combined.
Gradually add the dry ingredients and mix on low speed until just combined.
Fold in the oats gently using a spatula or wooden spoon.
Carefully fold in the chocolate chips and chopped Cadbury mini eggs, spreading them evenly without overworking the dough.
Drop 3-tablespoon scoops of dough onto the prepared baking sheet, spacing about 3 inches apart.
Bake for 8-9 minutes, until edges are golden and centers appear slightly underdone.
Remove from oven and immediately press the crushed Cadbury mini eggs onto the warm cookies.
Let cookies cool on the baking sheet for 10 minutes then transfer to a wire rack or plate to cool completely.