Heat oven to 350 degrees. Spray or butter an 8-inch square baking dish (or equivalent size) and set aside. (You will be serving straight from the dish, so use an attractive one that can go from oven to table.)
In a 12-inch skillet, heat 2 tablespoons butter until melted. Saute spinach and garlic for 5 minutes, then remove from heat and set pan aside.
In a medium bowl, combine eggs, milk, onion, mustard, basil and salt. Stir in ham, cheese and spinach. Pour mixture into prepared dish and bake, uncovered, for 35 minutes.
Remove from oven and serve straight from baking dish. Use cocktail knives to spread hot spinach dip onto toasted slices of bread.