1. Preheat your oven to 350°F. Generously spray a 9-inch quiche pan or deep pie dish with nonstick cooking spray, getting into all the corners. Place the greased dish on a baking sheet.

2. Start with the diced onion spread evenly across the bottom of the dish. This creates your flavor foundation.

3. Sprinkle the chopped spinach over the onions in an even layer. The spinach will wilt during baking without overwhelming the dish.

4. Arrange the halved cherry tomatoes cut-side up across the spinach. They’ll look beautiful and add bursts of sweetness in every bite.

5. Sprinkle the shredded mozzarella evenly over everything. The cheese creates pockets of creamy goodness throughout.

6. Top with the sliced mushrooms, arranging them decoratively since they’ll be visible in the final dish. Take a moment to make it pretty.

7. In a medium bowl, whisk together the eggs and heavy cream until well combined and slightly frothy. This incorporates air for a lighter texture.

8. Add the ground mustard, garlic powder, salt, and black pepper. Whisk again until all seasonings are evenly distributed.

9. Slowly pour the egg mixture around the vegetables, not directly on top. This technique helps keep your arranged vegetables in place. If any vegetables shift, use a fork to gently nudge them back into position.

10. Bake for 40-45 minutes until a toothpick inserted in the center comes out clean. The top should be golden and the center should be just set.

11. Let the frittata cool for 10 minutes before slicing. This resting period helps it set completely for cleaner cuts. Slice into wedges and serve with your favorite toppings.