Preheat the oven to 350 degrees Farenheit. Grease a 9 inch pie dish.
Heat a frying pan over medium heat. Melt the butter and then saute onion until transparent. Add the garlic and saute another minute. Add the spinach a little at a time and cook until it is all wilted. Place this mixture in the prepared pie dish.
In a medium bowl, whisk the eggs, milk, salt, and pepper. Pour this over the spinach mixture in the pie dish.
Sprinkle the cheese on top. Use a utensils to gently push the cheese into the eggs.