To make marinade, combine tomato paste and stock in small bowl. Add vinegar, Worcestershire sauce, tamari, garlic and scallions, stirring after each addition. Set aside.
Arrange tofu in single layer in non-reactive baking dish. Cover with marinade, and refrigerate for 1 hour, or overnight.
Preheat oven to 400F.
Bake tofu in covered baking dish 30 minutes. Pour marinade into saucepan, and brown tofu under broiler.
To make sauce, heat marinade for 2 minutes. Add yeast and oregano. In small bowl, combine cornstarch and water, and add to marinade mixture. Cook sauce for 3 minutes, and serve spooned over tofu.