Preheat oven to 375°F. In medium saucepan melt butter and add onions and garlic. Sautee until onions are slightly browned and tender. Stir in spinach and turn off heat. Add shredded cheese and stir until cheese is soft. Set aside.
In medium bowl add eggs and milk, whisk until light and foamy. Add spinach mixture to egg mixture, gently combine. Pour into pie crust.
Bake in oven for 25 min, sprinkle with small amount of shredded cheese and allow to bake for 15 min longer or until the middle of the pie is firm. Allow to set for 10 min before serving.