Preheat oven to 375°F Heat 1 tbsp of olive oil in a large skillet over medium heat. Add the onion and carrot, cover, and cook until tender, about 5 minutes. Add the mushrooms and cook, stirring occasionally, for 3 minutes. Stir in tomato paste, tamari, veggie stock, thyme, marjoram, and salt and pepper to taste. Stir in cornstarch mixture and simmer to thicken slightly, about 1 minute.
Spoon the filling into a lightly oiled 2.5 quart baking dish. Stir in the chopped burgers, peas and walnuts. Taste and adjust the seasonings. Spread the mashed potatoes over the top. Sprinkle with paprika and drizzle with 1 tbsp of olive oil.
Bake until the potatoes are hot and bubbly and the top is golden brown, about 30 minutes. Serve hot.