In a large skillet, heat 1 tablespoon of avocado oil over medium heat. Add the diced onions and sauté them for about 3-4 minutes until they become soft and translucent.

Once the onions are ready, add 1 pound of lean ground beef to the skillet. Cook for 8-10 minutes, breaking it apart with a spatula until it is browned and crumbly.

In the same skillet, add another tablespoon of avocado oil, followed by 8 ounces of chopped mushrooms. Sauté for 5 minutes until the mushrooms are softened.

Stir in 2 tablespoons of flour, allowing it to cook for about 1 minute, then gradually add 2 cups of low-sodium beef broth, stirring continuously until the sauce thickens.

Remove the skillet from heat and combine the onion-beef mixture with the mushroom sauce. Add 1 cup of cooked lentils, 2 cups of frozen mixed vegetables, and 1 cup of cauliflower rice.

Stir until well combined, and fold in half of the shredded cheddar cheese, spreading the mixture evenly into a greased baking dish.

Arrange a layer of tater tots evenly over the filling in the baking dish, then sprinkle the remaining shredded cheddar cheese over the tater tots.

Preheat your oven to 400°F (200°C). Once heated, place the casserole in the oven and bake for 35-40 minutes.

After baking, allow the casserole to cool for about 5 minutes before serving. Consider garnishing with freshly chopped parsley or sliced green onions.