Notes: This dish can be assembled (through step 4) up to 1 day ahead; cover and chill. Bring to room temperature before baking or increase baking time by 10 to 15 minutes. If you can't find escarole, choose another leafy green, such as spinach or chard.
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
How to Make It
Pour oil into a 10- to 12-inch frying pan over medium-high heat. Add onion and stir often until limp and slightly golden, about 5 minutes. Stir in garlic and crumble sausage into pan. Stir often until meat is no longer pink, 7 to 10 minutes, using a wooden spoon to break up the meat if necessary.
Add escarole to sausage mixture in pan and reduce heat to low. Stir until escarole is wilted, 1 to 2 minutes. Remove pan from heat.
In a large bowl, whisk the milk and eggs until combined. Stir in Gruyère, bread cubes, salt, pepper, and nutmeg.
Add sausage mixture and stir to mix well. Scrape mixture into a buttered 9- by 13-inch baking dish and spread level (see notes).
Bake in a 375° oven until top is golden and a knife inserted in the middle comes out mostly clean, 35 to 45 minutes. (If top is brown after 30 minutes, cover loosely with foil and continue to bake.) Serve hot.