Preheat your oven to 375°F (190°C). Gather your ingredients for assembly.

Melt 3 tablespoons of unsalted butter in a large skillet over medium heat. Add 4 cups of thinly sliced onions and cook until caramelized and golden brown, about 20-25 minutes.

Stir in 3 minced garlic cloves and sauté for 1-2 minutes. Pour in 1 cup of beef broth and let simmer for 5 minutes.

In a saucepan, heat 2 cups of heavy cream over medium-low heat. Whisk together 2 tablespoons of flour with 2 tablespoons of melted butter, then gradually whisk into the cream until thickened, about 5-7 minutes. Season with salt and pepper.

In a lasagna pan, spread a thin layer of béchamel sauce, then lay down 3-4 sheets of lasagna noodles. Add half of the caramelized onions, a layer of béchamel, and a sprinkle of cheese. Repeat layers, finishing with noodles topped by remaining béchamel and cheese.

Sprinkle croutons over the top layer of cheese for added crunch.

Bake the lasagna for 35-40 minutes until the cheese is bubbling and golden brown.

Let it cool for at least 10 minutes before serving.