Butter a 2qt baking dish. Place on large baking sheet (this prevents the dish from overflowing onto your oven bottom).
Heat up olive oil in large skillet. Add chopped onion, carrots and celery. Cook until onion is translucent and carrots are tender, about 5 minutes. Stir often. Season with salt and pepper.
Add fresh thyme, sliced mushrooms and garlic and saute until fragrant.
In a measuring cup, whisk together vegetable stock and cornstarch. Add tomato sauce and Worcestershire sauce and whisk. Add mixture to vegetables in skillet. Bring to boil, let simmer until thickens.
Pour mixture into prepared baking dish. Add 1 cup frozen peas and stir in.
Top with mashed potatoes, spreading them evenly.
Bake at 375℉ for 15 minutes, or until bubbly at the edge.