Prepare through Step 2 up to a day ahead; cover and chill. Remove from the fridge, add the buttered crushed croutons, and let stand 30 minutes before baking.
Preheat oven to 350°. Bring garlic salt and 4 cups water to a boil in a large saucepan over medium-high heat; gradually stir in grits. Reduce heat to medium, and cook, stirring often, 5 minutes or until thickened. Remove from heat, and stir in onion and 3 Tbsp. butter.
Whisk together eggs and next 5 ingredients in a large bowl. Stir about one-fourth of grits mixture gradually into egg mixture; add remaining grits mixture, stirring constantly. Pour into a lightly greased 13- x 9-inch baking dish.
Melt remaining 2 Tbsp. butter, and toss with coarsely crushed croutons; sprinkle over grits mixture.
Bake at 350° for 30 to 35 minutes or until mixture is set and croutons are golden brown.
Note: For testing purposes only, we used Oscar Mayer Real Bacon Bits.