Preheat oven to 375°F.
In a medium saucepan over medium-high heat add 2 TBSP of the butter, spinach and the diced onion. Saute for about 5 minutes or until any liquid has evaporated, stirring occasionally, add the garlic and cook for about 30 more seconds or until fragrant. Take off the heat and set aside.
In a large bowl mix the flour, salt and baking powder. Stir in eggs, milk and remaining butter. Mix in the cooked spinach and the Monterey Jack cheese.
Spread the mixture into a 9x13 baking dish evenly. Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean. Let rest 10 minutes, cut into squares and serve.
