Heat butter in a large skillet over medium heat; cook and stir onion, celery, garlic, and tofu until onion is tender and translucent, 10 to 15 minutes. Season with oregano, salt, and black pepper. Stir flour into vegetable mixture; toss to coat.
Slowly stir milk into vegetable mixture until thickened; about 5 minutes. Stir in vegetable broth; cook and stir until filling is thickened, about 5 minutes. Stir in frozen vegetables. Fit one pie crust into a 9-inch pie pan.
Pour filling into prepared pie crust. Top with second pie crust; crimp edges together to seal. Poke holes into top crust for ventilation during baking.
Bake pie in the preheated oven until crust is golden brown and filling is bubbling, 30 to 35 minutes. Let cool for 10 to 15 minutes before serving.
