Place onion, garlic and EVOO into the Thermomix bowl. Chop 4 sec/speed 5. Cook 5 min/Varoma/speed 1/MC off. Set aside in a large bowl.
Place potatoes, carrot and celery into the Varoma dish. Line the Varoma tray with wet, scrunched up paper and place the chicken on it.
Fill the Thermomix bowl with water to the 1 litre mark and set the Varoma into position. Cook 25 min/Varoma/speed 3. Check that everything is cooked through, add a few minutes cooking time if needed. Add to the bowl with the sauteed onion.
Add peas and combine.
To make the mustard bechamel, place all ingredients into the Thermomix bowl and cook 8 min/80/speed 4.
Cook 2 min/90/speed 4. Pour into the bowl with the chicken and vegetables. Mix it all together, taste and add extra seasoning if needed.
Preheat the pie maker.
Roll out the puff pastry thinly, cut circles to fit your pie maker, 2 sizes, the larger one goes in the bottom.
Place the larger pastry circles into the pie maker, fill with pie filling and place the smaller pastry circle on top. Brush with egg wash and cook for 10 minutes, or until browned.
Serve with homemade Fermented Ketchup! (Which doesn't sound that good, but it is DE-lish!)