Preheat oven to 190 degrees celsius (fan-forced). Grease a casserole dish and set aside.
Rinse the quinoa under cold water in a sieve and then place into a saucepan along with the water and vegetable stock powder. Cook for 10-15 minutes (at a gentle simmer) until all of the water has been absorbed. Remove from the heat, place the lid on and allow to rest for 5 minutes. Fluff up with a fork and place into the casserole dish.
Meanwhile, place the roughly chopped vegetables into the Thermomix bowl and finely chop for 5-10 seconds on Speed 8 (using the spatula to assist). Remove the vegetables from the Thermomix bowl and strain any excess liquid away.
Place the finely chopped vegetables into the casserole dish with the cooked quinoa and corn kernels.
Place the cheese in to the Thermomix bowl and grate for 10 seconds on Speed 10. Set aside.
In a clean Thermomix bowl, place the butter, milk, plain flour, salt, pepper and half of the grated cheese and cook for 7 minutes, 80 degrees, Speed 4.
Pour the cheese sauce over the vegetables and quinoa and stir to combine.
Sprinkle over the remaining grated cheese.
Bake in the oven for 30 minutes or until the cheese is golden on top and vegetables are cooked through.