Preheat oven to 425 degrees. Butter a 9 inch deep dish pie pan.
Heat 1 tablespoon olive oil in a skillet over medium heat. Add in chard and minced garlic. Cook just until chard wilts, 2 - 3 minutes. Season to taste with salt and pepper. Transfer to a plate and set aside.
In the same skillet heat remaining tablespoon of olive oil and butter over medium heat until butter melts. Stir in onions and mushrooms. Cook until they are soft and begin to caramelize, about 10 minutes. Sprinkle the flour over the top, cook one minute. Slowly whisk in broth, slowly whisk in milk. Cook until it thickens slightly, about 3 minutes. Whisk in cajun seasoning. Season with salt and pepper as needed. Remove from heat.
Layer sliced andouille sausage on the bottom of the prepared pie pan.
Combine pepper jack and mozzarella cheese together in a bowl.
Now that the chard has had a chance to cool, use paper towels to squeeze out the excess water from the chard. Layer the chard over the top of the sausage. Pour the mushroom gravy over the top. Sprinkle 1/2 of the cheese over the top. Layer tater tots on top. Bake in preheated oven for 25 minutes. Remove from oven, top with remaining cheese. Return to oven and bake for 10 more minutes or until cheese is melted, edges are bubbly and tater tots are brown and crispy.
Allow to set for 5 minutes. Serve.
Notes
It is really important to squeeze out the excess water from the chard. If there is still water, it will continue to release it as it cooks and the final dish will be a bit watery. You can also make this in a 9x13 casserole dish, you'll just need more tater tots. Nutrition information for estimation purposes only.