Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Combine corn and eggs in a small bowl, stirring with a whisk; pour into pan. Loosen omelet with a spatula, tilting pan to allow uncooked egg mixture to come in contact with pan. (Mixture will puff and bubble.) Cook 2 minutes or until browned on bottom. Cut omelet into quarters, using a spatula. Turn each quarter over; cook until set.
Transfer omelet to a cutting board; cut into 1-inch pieces.
Heat remaining 1 1/2 teaspoons oil in a Dutch oven over medium-high heat. Add 1 cup onion and mushrooms; sauté 4 minutes. Add Vegetarian Dashi and the remaining ingredients except green onions. Reduce heat; cook 4 minutes or until spinach wilts. Stir in omelet pieces. Garnish with green onions, if desired.