COMBINE the mushrooms and allow to sit loosely packed in a tall, airtight container (preferably a quart container. In a small saucepan, combine the sugar, water and vinegar. Bring to a boil, whisk to ensure the sugar is dissolved and remove from the heat. Allow the pickling liquid to completely cool and pour over the mushrooms. Add a small weight on top of the mushrooms to ensure they are completely submerged, cover with a lid and allow to pickle in the refrigerator for 1 week. Drain before serving.
MAKE the blanquette, once the mushrooms are pickled. In a medium saucepan, bring the mushroom stock to a boil. Reduce the heat to a simmer and cook until the liquid has reduced to 1 cup, 15 to 20 minutes. Transfer the reduced mushroom stock to a blender. While the motor is running, add the remaining ingredients, replace the lid and blend on high for 2 to 3 minutes. Transfer the blanquette back to the saucepan and warm over low heat, 1 to 2 minutes. Don't turn up the heat too high or the mixture will overcook and the eggs will scramble.
TRANSFER the toast to serving plates, add the pickled mushrooms, spoon over the blanquette, sprinkle with thinly sliced chives and serve.