Preheat oven to 400º.
Toss scapes, asparagus and olive oil in a mixing bowl. Add salt and pepper to taste. Spread evenly over a baking sheet and roast for 15 minutes until the vegetables are tender but not brown. Remove from oven, toss with two teaspoons lemon juice and set aside to cool.
In a small saucepan, bring the stock to a simmer. Crack the egg and gently slide it into the simmering broth. Poach the egg until the whites firm up but the yolk is still soft. Pour the stock and egg into a blender with the mustard and vinegar. Puree until smooth. With the blender running, drizzle in the oils until well emulsified. Add salt and pepper to taste.
Toss the mizuna with the remaining lemon juice and put in a shallow platter or bowl. Add the scapes and asparagus over the top and drizzle half the vinaigrette over the vegetables, reserve the rest.
Serve this asparagus salad with a few garlic crostinis.