Begin by preheating your oven to 375°F (190°C).

Grease a 9x13 inch baking dish with cooking spray or olive oil. Arrange the frozen tater tots in an even layer on the bottom of the dish, forming a crust. Bake the tater tots in the preheated oven for about 20-25 minutes, or until they are golden brown and crispy.

While the tater tots are baking, heat a drizzle of olive oil or butter in a large skillet over medium heat. Add the diced onions and sliced portobello mushrooms to the skillet. Cook, stirring occasionally, until the vegetables are softened and lightly browned, about 5-7 minutes. Add the chopped spinach to the skillet and cook for an additional 2-3 minutes until wilted. Season with salt and pepper to taste.

In a mixing bowl, whisk together the eggs and heavy cream until well combined. Season with oregano, salt and pepper.

Once the tater tot crust is ready - use the bottom of a glass to smush them down to compact it a little. Spread the sautéed vegetables evenly over the top. Pour the egg mixture over the vegetables, ensuring it covers them evenly. Sprinkle the grated fontina cheese over the top of the casserole.

Place the assembled casserole back into the oven and bake for 25-30 minutes, or until the eggs are set and the cheese is melted and bubbly.

Once baked, remove the casserole from the oven and let it cool for a few minutes before slicing. Serve warm, garnished with fresh herbs if desired.