Preheat the oven to 375°F (190°C). Lightly grease a 9 × 13 inch baking dish.
Heat the olive oil in a large skillet over medium heat. Add the onion and celery and sauté for 5–7 minutes, until soft and lightly golden. Add the garlic and cook for about 30 seconds, until fragrant.
Stir in the ground cumin, smoked paprika, dried thyme, black pepper, and salt. Add the cooked lentils and corn and cook for 2–3 minutes, stirring well, until everything is evenly seasoned and warmed through.
Spread the lentil mixture evenly in the prepared baking dish and gently press it into an even layer.
Make the creamy vegan mushroom sauce:
In the same skillet, melt the vegan butter over medium heat. Add the chopped mushrooms and cook for 6–8 minutes, stirring occasionally, until they release their moisture and begin to brown.
Sprinkle the flour evenly over the mushrooms and stir well. Cook for 1 minute to remove the raw flour taste.
Slowly pour in the plant milk, stirring constantly. Bring to a gentle simmer and cook for 4–6 minutes, until thick and creamy.
Season with salt, black pepper, garlic powder, Dijon mustard, apple cider vinegar, and nutritional yeast. Taste and adjust seasoning as needed. Remove from heat.
Assemble the casserole and bake:
Pour the warm mushroom sauce evenly over the lentil layer, spreading gently to fully cover it.
Sprinkle the shredded vegan cheddar cheese evenly over the sauce.
Arrange the tater tots on top in a single, even layer. Lightly spray with oil or drizzle with 1–2 teaspoons neutral oil to help them crisp during baking.
Bake uncovered for 40–45 minutes, until the casserole is bubbling around the edges and the tater tots are deeply golden and crisp.
For extra crispiness, broil for 2–3 minutes at the end, watching closely.
Let the casserole rest for 10 minutes before serving.
Notes
Refrigerator: Store leftover tater tot casserole in an airtight container in the refrigerator for up to 4 days.
Freezer: This casserole can also be frozen for up to 2 months, either before or after baking.
To reheat: Reheat in the oven or air fryer to keep the tater tots crisp. I recommend thawing frozen tater tot casserole in the fridge for quicker and more even reheating.