Mix the rolled oats with the first 50 ml water and leave them to soak for 10 minutes.
Peel and coarsely grate the onion.
Whisk the eggs with 30 g yoghurt, salt, and thyme.
Add the onion and egg mixture to the soaked oats.
Gradually add up to 50 ml additional water. The mixture should be thick but easy to spread.
Heat 10 ml oil in a 24 cm non-stick pan over medium-low heat.
Spread the oat mixture evenly in the pan. Cover and cook for 6–8 minutes.
Once the underside is golden and the top is mostly set, turn the base using a large plate.
Sprinkle 100 g mozzarella over the cooked side.
Cover and cook over low heat for another 5–7 minutes, until the bottom is golden and the cheese has melted.
Grate the cucumber, lightly salt it, leave for five minutes, and squeeze out the liquid.
Mix the cucumber with yoghurt, Dijon mustard, crushed garlic, pepper, and optional lemon juice and herbs.
Rest the oat skillet for two minutes, cut it into four wedges, and serve with the sauce.