This tasty casserole has all the flavors of French onion soup in the form of a comforting, scoopable side dish. Serve it alongside a roast for a special dinner, or with eggs at brunch. No matter what time of day you eat it, this crowd-pleaser is easy enough to become a standard in your meal planning.
In a large pot, melt 2 tablespoons of the butter over medium heat. Add the oil and onions and toss well to coat the onions. Season with salt and pepper, add the bay leaf, and continue to cook, stirring occasionally, until the onions soften, 5 to 7 minutes.
Reduce the heat to medium low and continue to cook, uncovered, stirring occasionally, until the onions begin to caramelize, 25 to 30 minutes longer.
While the onions cook, make the bechamel sauce: Melt 2 tablespoons of the remaining butter in a small saucepan over medium heat. Add the flour and whisk to combine. Whisk in the milk and bring to a simmer, whisking. When the sauce thickens, stir in the Parmesan and nutmeg. Season with salt and pepper. Set aside.
Preheat the oven to 400 degrees F. Grease a 9-by-13-inch baking dish with the remaining 1 tablespoon butter.
When the onions begin to caramelize, stir in the balsamic and Worcestershire and cook for 1 to 2 minutes more. Add the broth and thyme and bring to a simmer over medium heat. Remove and discard the bay leaf.
Transfer the onion mixture to the prepared dish. Arrange the baguette slices on top, then spoon a little bechamel sauce over each piece of bread. Top with the mozzarella. Bake until the cheese is melted and begins to brown and the filling is bubbling, 35 to 40 minutes.
Let cool for 5 to 10 minutes. Sprinkle with chives and serve.