Heat the oil in the saucepan, add the vegetables and sauté over medium heat, stirring frequently, for 5 minutes. Add garlic and continue sautéing for another 5 minutes.
Sprinkle flour on top and continue cooking, stirring constantly, for 2 minutes.
Gradually add the warm liquid, stirring constantly, until sauce thickens.
Add thyme and parsley and continue cooking until all of the vegetables are tender, about 10 minutes. Stir in salt and pepper, then transfer vegetables to the baking dish.
Preheat oven to 400.
Make the pie crust dough, then roll it into a circle slightly larger than the casserole. Lay on top of the vegetables, without sealing the edges. Slash the crust a few times.
Place in oven and bake until filling is bubbling and crust is golden brown, 20-30 minutes.
*For a smaller version with a higher crust-to-filling ratio, cut all the incredients in half (except the pie crust) and make the pot pie in a pie plate.
This recipe is very flexible. You can use just one or two vegetables, or a whole assortment. You can make it chunkier by increasing the amount of vegetables, or saucier by increasing the amount of liquid. Try it my way, and then experiment to discover your own favorite.