French Onion Pasta Bake

A cozy, flavorful French onion pasta bake includes sweet caramelized onions, sauteed mushrooms, fresh spinach, and plenty of cheese!

Course Dinner

Cuisine American

Keyword french onion pasta, french onion pasta bake, french onion pasta casserole, french onion pasta recipe

Prep Time 15

Cook Time 1 10

Total Time 1 25

Servings people

Calories 414kcal

Author Blair Lonergan

¼ cup (half of a stick) salted butter
1 ½ lbs. Vidalia onions (about 1-2 large onions), peeled and sliced
8 ounces sliced fresh mushrooms
1 large clove garlic, peeled and minced (about 1 teaspoon)
¼ cup all-purpose flour
1 teaspoon minced fresh thyme (or ¼ teaspoon dried thyme)
2 ½ cups beef broth
1 tablespoon Worcestershire sauce
1 bay leaf
1 tablespoon balsamic vinegar
Kosher salt and ground black pepper, to taste
1 (16 ounce) package rigatoni or penne pasta, cooked in a large pot of salted water according to package directions for al dente
2 cups coarsely chopped baby spinach
6 ounces (about 1 ½ cups) shredded Swiss cheese, Gruyere cheese, or mozzarella cheese
Optional garnish: chopped fresh parsley or thyme
Optional, for serving: sliced baguette

Preheat oven to 375°F. Grease a 9 x 13-inch baking dish with nonstick spray; set aside.
Melt the butter in a large Dutch oven over medium heat. Add the onions and saute for 20 minutes, stirring frequently. Add the mushrooms and garlic; saute until mushrooms are tender, about 5-7 more minutes.
Sprinkle in the flour and cook, stirring constantly, for 2 minutes. Add the thyme and about half of the beef broth. Deglaze the pan, stirring to loosen the cooked pieces on the bottom. Whisking constantly, add the remaining beef broth, Worcestershire sauce, and bay leaf. Increase the heat to medium-high and bring to a boil. Reduce the heat to low and simmer for 4-5 minutes, until the sauce thickens slightly. Stir in the balsamic vinegar and season with salt and pepper to taste. Discard bay leaf.
Stir in the cooked pasta and the spinach. Transfer to the prepared baking dish. Top with shredded cheese. Cover with foil.
Bake covered for about 20 minutes, until the filling is hot and bubbly. Remove the cover, and place the pan under the broiler for about 1 minute, or until the cheese is browned and crisp on top. Garnish with fresh herbs, if desired.

Stir the onions regularly and add water to the pan as needed to ensure that the onions don’t burn. If they start to get too dark too quickly, just lower the temperature and cook them over medium-low or low heat.

Boil the pasta just until it’s barely al dente (just about tender, but still with a firm bite). The rigatoni will continue to cook in the oven, and you don’t want it to come out mushy, gummy, and overdone.

Bake the casserole covered so that it stays moist and doesn’t dry out. Then, for a crispy top, place the dish under the broiler for a minute or two before serving.

Keep a close eye on the dish while it’s under the broiler, since the cheese can burn quickly.

Garnish each serving with fresh herbs for a bright, colorful touch. Good options include fresh thyme leaves, parsley, rosemary, or chives.