2 tablespoons olive oil
1 onion (10 oz.), peeled, cut in half, and thinly sliced crosswise
2 cloves garlic, peeled and minced or pressed
1 pound fresh mild or hot Italian sausage (about 4 links), casings removed
4 ounces escarole (see notes), rinsed, tough white parts removed, and thinly sliced
2 cups whole milk
4 large eggs
1 cup shredded Gruyère or Swiss cheese
10 cup 1- to 2-inch cubes French or country-style bread (1 lb.; crusts removed)
1 teaspoon salt
1/4 teaspoon pepper
1/8 teaspoon fresh-grated nutmeg