1. Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking dish.
2. Steam the broccoli florets for about 4-5 minutes until they’re tender but still bright green. Drain and set aside.
3. In a large skillet, heat the olive oil over medium heat and add the sliced onions. Sprinkle with sugar and salt, and cook for about 25-30 minutes, stirring occasionally, until they’re caramelized and golden brown. This step requires patience but trust me, it’s worth it.
4. In a separate saucepan, melt the butter over medium heat. Stir in the flour and cook for 1-2 minutes, creating a roux. Slowly whisk in the milk and broth, continuing to whisk until the mixture thickens, about 5-7 minutes.
5. Remove the saucepan from heat and stir in the cheddar and Gruyère cheeses until melted and smooth. Add the Dijon mustard, and season with salt and pepper to taste.
6. In your prepared baking dish, combine the broccoli and caramelized onions. Pour the cheese sauce over the top, ensuring everything is evenly coated.
7. Sprinkle the French fried onions over the casserole, adding that perfect crunchy topping.
8. Bake in the preheated oven for 20-25 minutes, until the top is golden and bubbly. Let it cool for a few minutes before serving.

This dish is a tribute to the many nights spent in my family’s kitchen, watching my mother transform simple ingredients into something extraordinary.