Preheat oven to 350 degrees F / 177 degrees C.
Heat a large skillet pan over medium heat. Add 1 tablespoon butter and melt. Add 2 ounces mushrooms and cook for about 3 minutes until brown, stirring occasionally.
1 tablespoon butter, 2 ounces mushrooms
Remove from heat, reserve, and cool.
Place 6 eggs and ½ cup heavy cream in a medium bowl and whisk together.
6 eggs, ½ cup heavy cream
Add 2 ounces cream cheese to mixture and break it up slightly with a whisk. Then add 1 cup ham, ½ cup Swiss cheese, ¼ teaspoon sea salt, and ¼ teaspoon black pepper. Stir to combine.
1 cup ham, ½ cup Swiss cheese, ¼ teaspoon sea salt, ¼ teaspoon black pepper, 2 ounces cream cheese
Add cooked mushrooms to cheese mixture.
Pour mixture into an 8x8-inch prepared baking dish and evenly distribute ingredients.
Cover with foil and bake for 30 minutes.
Check for doneness. The edges should be set, and the center not quite cooked.
Bake uncovered, if needed, for an additional 5 to 8 minutes until casserole is uniformly golden brown and middle is set.
Cool for 3 to 5 minutes before serving.