Bring about 1/2 inch of water and 1/2 teaspoon salt to boiling in a medium-size saucepan. Add the asparagus and half of the green onion. Simmer, covered, 5 minutes or until just tender. Drain. Combine asparagus mixture with sour cream in a bowl. Reserve.
Heat butter in a 10-inch nonstick skillet** over medium-low heat. Add remaining half of green onion; sauté 2 minutes or until just tender.
Beat eggs, water, 1/2 teaspoon salt and pepper in a bowl. Add to skillet. Using a spatula, lift cooked mixture, letting uncooked mixture flow underneath. When cooked, omelet should be barely brown on bottom, moist in center. Spoon asparagus filling over half of omelet; fold over the other half. Serve immediately.