Heat the oven to 375°F (190°C). Lightly coat a baking dish with nonstick spray.
Warm a skillet over medium heat, then cook the diced onion for about 5 minutes, stirring occasionally, until tender and translucent.
In a large mixing bowl, add the cottage cheese, cooked onion, chopped spinach, eggs, garlic powder, salt, pepper, and half of the shredded cheddar. Stir until the mixture looks evenly blended.
Spoon the egg mixture into the prepared baking dish and spread it out evenly. Scatter the rest of the cheddar across the top.
Place the dish in the oven and bake for 30 to 35 minutes, or until the center feels firm and the surface turns lightly golden.
Remove from the oven and let it sit for a few minutes before cutting into slices.