Grease eight 10-ounce individual casseroles; set aside. In a large skillet cook mushrooms, shallots, and garlic in hot oil over medium heat about 5 minutes or until tender. Carefully add sherry. Simmer, uncovered, until liquid is nearly evaporated. Transfer mixture to a large bowl. Stir in focaccia pieces.
In a medium bowl combine eggs, half-and-half, cheese, thyme, rosemary, salt, and pepper. Pour egg mixture over focaccia mixture, pressing with a wooden spoon to moisten all of the bread.
Divide mixture among the prepared casseroles. Cover and chill for 2 to 24 hours.
Preheat oven to 325 degrees F. Bake, uncovered, for 35 to 40 minutes or until a knife inserted near the centers comes out clean.