- 1 tablespoon butter, plus more for greasing the pan
- 1 small yellow onion, chopped (about 1 cup)
- 1/4 teaspoon kosher salt
- Freshly ground black pepper
- 8 large eggs
- 1/3 cup milk
- 4 cups bread cubes (about 1-inch) cut from a hearty loaf, such as a baguette
- 1 1/2 cups (about 4 ounces) shredded Fontina cheese
- 3/4 cup fresh basil, chopped
In a saucepan over medium-high heat, heat 1 tablespoon butter until foamy, then add the onion and season with 1/8 teaspoon of salt and a few grinds of pepper. Cook, stirring often, until the onion is softened, 4-5 minutes.
Transfer the onion to a heatproof mixing bowl. Whisk in the eggs, milk, remaining 1/8 teaspoon of salt, and pepper.
Grease a 9-inch square glass baking dish with butter. Add half of the bread cubes in a layer on the bottom of the dish. Top evenly with half the fontina, then half the basil. Pour on half the egg mixture. Repeat the layering once, starting with the remaining bread cubes. Then toss all gently in the pan to help thoroughly coat the ingredients with the egg. Cover with plastic wrap and refrigerate at least 1 1/2 hours, or overnight, to help the bread absorb the egg mixture. Let stand at room temperature 30 minutes before baking.
Position a rack in the center of the oven and preheat the oven to 325°F. Bake until the top of the strata is lightly browned and the middle no longer looks runny when pierced with a knife, about 45 minutes. Serve warm, cut into squares.