Combine the ginger, garlic, and curry paste in small bowl. Slowly pour in 1/4 cup coconut milk; stirring until the mixture is smooth. Gently stir in the seitan, coating the seitan with the marinade. Refrigerate for 30 minutes to 1 hour. The longer you let it soak, the more flavorful the seitan will taste!

Stir together the remaining coconut milk, peanut butter, sugar, vegetable oil, soy sauce, chili garlic sauce, and oyster sauce in a medium bowl. Don't worry if it's not completely blended: when you cook it, the various substances will meld wonderfully. Set sauce aside.

Lightly coat a large skillet with cooking spray; add the chopped onion; cook over high heat for 3 to 4 minutes, or until the onion is tender. Stir in the seitan, and cook until the seitan is heated through, about 7 minutes. Pour in the sauce, and stir to combine. Mix in the spinach and green onions; cook 3 minutes, or until the sauce has thickened and the spinach is cooked.