Cut spinach into 2cm lengths.

Slice the onion thinly.

Heat butter in a pan, then sauté onion and spinach.

Sprinkle flour over the mixture and stir until no longer powdery.

Add milk, salt, and pepper, then mix until thickened. Stir in 2 tbsp of pizza cheese.

Slice chicken breast horizontally to open it up, then flatten with the back of a knife.

Place the cooled filling onto the chicken and fold to enclose.

Place stuffed chicken on a baking sheet, season with salt and pepper, then top with remaining cheese and breadcrumbs.