Growing up, this was my favorite. I was constantly pestering my mom to cook it.

Prepare your oven for baking at 350°F (175°C).
Place the olive oil and butter in a large skillet and heat over medium heat. Cook, stirring occasionally, the sugar and sliced ​​onions until the onions caramelize and turn golden brown. Estimated time needed: fifteen to twenty minutes.
In a greased 9x13-inch baking dish, spread the uncooked rice evenly while the onions caramelize.
Season with salt and pepper and add the beef broth to the pan when the onions are caramelized. After mixing well, reduce the heat to low and let simmer.
Once the rice is coated in the baking dish, pour the onion and broth mixture over it.
Place the baking dish in the preheated oven and cover with foil. Bake for 30 minutes.
Return to the oven after 30 minutes, remove the foil and top with grated Gruyere cheese. For melted, bubbly cheese, bake uncovered for another 15 minutes.
To allow all the flavors to blend, remove from the oven and let sit for a few minutes before serving.
Tips and Variations
Swapping out the beef broth for vegetable broth will make this vegetarian-friendly. For a more rustic taste, try sautéing some mushrooms. Also, add some crispy fried onions on top of the meal just before serving if you're a fan of crunch.