Cut the potatoes into 7mm slices and soak in water. Thinly slice the onion and separate the broccoli into florets.

Microwave the potato slices in a heatproof container covered with plastic wrap for 5 minutes at 600W.

Steam the broccoli with 3 tbsp of water in a pan over medium heat for 2 minutes, then remove the lid and evaporate excess water.

Add olive oil and onion to the pan and sauté until the onion softens. Stir in flour and cook until no longer powdery.

Add potatoes, tuna, and the white sauce ingredients to the pan. Stir and cook until thickened.