Bring quinoa and salted vegetable stock to a boil. Immediately lower to a simmer, and cook for about 15-20 minutes, until most of the water has been absorbed and grain is tender. Fluff, strain, and set aside to cool.
Over medium heat, sauté the mushrooms in the butter and season with salt and pepper.
Once cooked, toss the mushrooms in the mustard.
To serve, divide quinoa amongst 4 plates. Spoon the mushrooms on the quinoa on one side, spilling on to the rest of the plate. Garnish with the nasturtium and radish slices. Carefully separate the yolk from the rest of the egg and place in the center of the quinoa (yes, raw!). Season with salt and pepper. Serve with crusty grilled bread.