Preheat oven to 425°F. Set rack in the center of the oven.
In a heavy skillet, melt 1 1/2 tablespoons of olive oil and sauté the onion with a pinch of salt over medium-low heat until translucent. Increase the heat, add the mushrooms and sauté for about two minutes. Add soy sauce and continue to sauté until the mushroom liquid is evaporated. Set aside.
Heat remaining oil and butter over low heat and sprinkle the flour on top. When the flour starts to bubble, whisk it to create a roux. Whisk continuously for about 2 minutes. Add the vegetable stock while whisking and bring to a boil. Continue to whisk. Add almond milk, whisk and allow to simmer for about 5 minutes. Add the reserved mushroom mixture, lentils, green beans, corn, peas, and red bell pepper. Season to taste with salt and pepper.
Arrange the tater tots on top and place in the oven. Lightly cover will foil and cook for about 45 minutes. Remove foil, lower oven temperature to 350°F and cook for another 15 minutes until the filling is bubbling, the lentils are fully cooked and the tater tots are golden brown.
Serve immediately and enjoy!
While this recipe relies on pantry- and refrigerator-staples (I like to keep a bag of tater tots, mixed vegetables, and green beans at the ready), feel free to toss in any fresh produce you have! Have some zucchini? Red bell peppers? Add them to the casserole, this hot dish is very flexible.