Heat the vegetable or canola oil in a large skillet over medium heat. Add the finely chopped sweet onion and sauté for about 2-3 minutes until it becomes translucent. Toss in the zucchini and cook for an additional 3-4 minutes until softened, then add the frozen hash browns and cook until golden brown and crispy, approximately 5-7 minutes. Stir occasionally.
In a mixing bowl, crack the 6 large eggs and pour in the 1/2 cup of milk. Whisk together until well combined and fluffy. Season with salt and pepper to taste.
Reduce the heat to low, and pour the egg mixture evenly over the hash brown and vegetable mixture in the skillet. Cook without stirring for about 5-7 minutes until the edges start to set.
Sprinkle the chopped tomatoes and shredded cheddar cheese over the top of the frittata. Transfer the skillet to the preheated oven and broil for about 3-5 minutes, or until the cheese is bubbly and golden.
Let the frittata cool for a few minutes before slicing. Serve warm or at room temperature.
