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1
Butter a 13 x 9 inch baking dish.
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2
Using 3 tablespoons butter, lightly butter all the bread slices on one side.
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3
Spread the mustard on top.
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4
Arrange half of the bread slices, mustard side up, in the baking pan in one layer; cut to fit if necessary.
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5
Heat 1 tablespoon butter in a skillet over med-low heat; add in the onion; saute for 25-35 minutes, stirring often, until the onion is softened.
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6
Spread the onion evenly over the bread slices.
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7
Top with half of the cheese and all of the ham.
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8
Cover with the remaining cheese and then the remaining bread slices; press the layers together.
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9
Beat the eggs, milk, basil, and salt together in a bowl; pour evenly over the layers, pressing them down slightly so they absorb the mixture.
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10
Cover and refrigerate for several hours or overnight.
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11
Remove from the refrigerator 1 hour before baking.
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12
Preheat oven to 375.
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13
Mix the breadcrumbs with the remaining 4 tablespoons butter and the parmesan cheese; sprinkle over the casserole.
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14
Bake, uncovered, for 35-40 minutes, until the top is golden brown and the egg mixture is set.
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15
Remove from the oven and let rest for 10 minutes before serving.
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16
To serve, cut into squares.
