Preparation Steps:

• In a large skillet, heat the 1 tablespoon of olive oil over medium heat. Add the 1 chopped onion and 1 chopped celery stalk. Sauté for about 3-4 minutes until they start to soften.
• Next, add the 5 chopped carrots and 12 oz. of sliced mushrooms to the skillet. Cook for another 5-7 minutes, stirring occasionally, until the vegetables are tender and the mushrooms have released their moisture.
• Sprinkle in the 1 teaspoon of fresh thyme, 1 teaspoon of salt, 1/2 teaspoon of black pepper, and the 1 tablespoon of minced garlic. Stir well to combine and cook for an additional minute until fragrant.
• Pour in the 1 1/2 cups of vegetable stock and 1 cup of tomato sauce. In a small bowl, mix the 1 tablespoon of cornstarch with a little cold water to create a slurry, then add this to the skillet to thicken the mixture. Stir everything together and let it simmer for about 5 minutes. Finally, add the 1 cup of frozen peas and mix well.
• In a casserole dish, spread the vegetable filling evenly.
• Spoon the 2 cups of mashed potatoes over the vegetable filling, spreading it out with a spatula. You can create peaks and valleys for a bit of texture.
• Preheat your oven to 400°F (200°C). Place the assembled casserole dish in the oven and bake for about 25-30 minutes, or until the top is golden brown and slightly crispy.