In a large heavy bottomed pan or skillet, heat the oil. Add the bell peppers, onions and celery; sauté for 3 to 5 minutes, stirring frequently.
Add the garlic, carrots and potatoes. Pour in the vegetable broth, Italian seasoning, peas and bring to a boil. Lower the heat and let it simmer until the vegetables are just tender, approximately 5 minutes. Season with salt and pepper.
In a small bowl, mix the water, cornstarch, and soy sauce until the cornstarch is fully dissolved. Stir this mixture into the vegetables and cook until the sauce thickens, about 3 minutes.
Transfer the filling on a 11 inch round baking dish. Roll out the dough and place it over the filling, sealing and fluting the edges.
Bake in the preheated oven until the top of the pie is golden brown, about 30 minutes.
Notes
Use a heavy-bottomed pan to evenly distribute heat, which helps prevent the filling from burning and ensures even cooking.
Stir in the garlic last, cooking for just a minute, to release its aroma without burning, which can turn the flavor bitter.
When adding the vegetable broth and peas, bring the mixture to a gentle boil before reducing the heat to simmer. This helps the vegetables cook evenly.
Dissolve the cornstarch completely in water before adding it to avoid lumps and ensure a smooth, thick sauce.
Roll out the pie pastry to cover the filling completely, crimping the edges to prevent the sauce from leaking during baking. Cut a few small slits in the pastry to allow steam to escape, preventing the crust from becoming soggy.