Cheesy Hashbrown Casserole

I made this, as a "breakfast for dinner" recipe.  It was loved by all!  Also, it was super easy!

Ingredients for Breakfast Casserole:  2 Tbsp butter, divided
1 small onion (1/2 cup) finely chopped
4 medium mushrooms, sliced (I omitted, as Ace doesn't like mushrooms.)
1 Tbsp olive oil
1 lb frozen hashbrowns
6 large eggs
1/4 cup sour cream
1/2 tsp Mrs. Dash salt-free seasoning, or your favorite seasoning
1/4 tsp sea salt and 1/8 tsp pepper, or to taste
1 cup shredded Mexican cheese, divided
To Garnish:6-8 baby red tomatoes, halved
Cilantro or Fresh chives for garnish

Instructions

1. Pre-heat oven to 350F. Heat your skillet over medium heat. Melt in 1 Tbsp butter then add chopped onion and saute until golden. Remove onion to a plate. Add sliced mushrooms to the hot skillet and saute until golden and soft. Remove to the same plate as the onions.
2. In the same pan over medium heat, melt 1 Tbsp butter with 1 Tbsp olive oil. Layer the hash browns evenly into the bottom of the pan and sauté until golden on the bottom (about 7 minutes total). Flip hash browns over a few times while sautéing. Once they are golden brown and nearly cooked through, remove from heat. Note: If you are baking in a casserole dish, transfer your potatoes to that dish and set aside.
3. In a large bowl, whisk together 6 large eggs, ¼ cup sour cream, ½ tsp seasoning along with about ½ tsp salt and ⅛ tsp pepper. Whisk together until well combined then stir in sautéed onion and mushrooms. Stir in ½ cup cheese just until combined.
4. Pour the egg mixture evenly over the hashbrowns and top with remaining ½ cup shredded cheese. Cover loosely with foil and Bake at 350˚F for 25 minutes then remove foil and Bake another 5-7 minutes or until cheese is bubbly and starting to brown. Garnish with halved baby tomatoes and fresh cilantro or sliced green onions.