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1
Arrange dry bread slices into a large shallow dish; pour cream all over and leave to soak.
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2
Into a large deep skillet, over medium heat, melt 3/4th of butter; soften leek slices, for approximately 8 minutes.
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3
Add mushroom strips; cook for 5 minutes more.
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4
Add Prosciutto strips; cook for 2 to 3 minutes longer.
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5
Reserving 1 tablespoon [15 mL] chopped parsley, stir remaining chopped parsley and fresh or dried thyme into mixture; set aside.
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6
Gently squeeze moisture from soaking bread, reserving liquid into a bowl [do not worry if bread breaks up].
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7
Grease a shallow 12 x 8-inch [3-L] baking dish; arrange 1/3 of bread slices over bottom or dish, top with 1/2 of leek mixture, then with 1/3rd of shredded cheese.
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8
Repeat layers once.
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9
Top with remaining bread slices, but do not worry if bread does not cover it all.
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10
Add eggs and mustard to reserved chopped parsley mixture, pepper then whisk until well combined; pour into baking dish.
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11
Sprinkle all over with remaining shredded cheese; dot with remaining butter.
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12
Sprinkle all over with reserved chopped parsley; tightly cover and refrigerate overnight.
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13
Preheat oven to 325u00b0F [160u00b0C].
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14
Bake, uncovered into preheated oven for 35 to 45 minutes, until top is golden brown and a knife comes out clean from its center.