In a medium pan, sauté the sliced mushrooms with a little bit of olive oil or butter until they are nice and toasty. Remove from the heat and set aside.
In a large pot on medium-high heat, toast the quinoa in butter (or oil), stirring regularly, until fragrant (about 4 minutes)
Add slices spring onion and sauté́ until softened.
Turn down the heat and add the stock cube that has been dissolved in two cups of hot water. Stir every few minutes, making sure the quinoa does not dry up. If it does, add a little bit more water until it is cooked through.
Just before all the liquid is gone, add the mushrooms, lemon juice, zest, and nutritional yeast (if using). Stir well for a few minutes, and then remove from the heat.
Serve immediately with extra spring onion and season to taste.