Preheat your oven to 350°F (175°C).

In a large ovenproof skillet, heat olive oil over medium heat. Add the diced onion and cook until translucent, about 3 minutes.

Stir in the chopped spinach and cook until wilted, about 2 minutes.

In a large bowl, whisk together the eggs, salt, and pepper. Add the cooked chicken and cheddar cheese to the egg mixture.

Pour the egg mixture over the vegetables in the skillet, ensuring it is evenly distributed.

Cook on the stovetop until the edges begin to set, about 4 minutes. Then transfer the skillet to the preheated oven and bake for 15-20 minutes, until the frittata is fully set.

Remove from oven, let cool for a few minutes, slice, and serve warm.