In a large skillet over medium-low heat, melt butter and olive oil. Add sliced onions and cook slowly for 30-40 minutes, stirring occasionally, until deeply golden brown and jammy. Add minced garlic and thyme, cooking for 1 more minute until fragrant.

While onions cook, bring a pot of salted water to a boil and cook orzo until al dente. At the same time, season chicken cubes with salt and pepper and cook in a separate skillet over medium-high heat until golden and cooked through.

Pour the beef broth into the skillet with the caramelized onions to deglaze, scraping up any browned bits. Bring to a simmer. Reduce heat and stir in the heavy cream and 1 cup of the shredded Gruyère cheese. Stir until the sauce is smooth and creamy. Season with salt and pepper.

Preheat oven to 375°F (190°C). In a large bowl, combine the cooked chicken, drained orzo, and the creamy onion sauce. Mix well.

Transfer the mixture to a 9x13 inch baking dish. Sprinkle the remaining 1 cup of Gruyère cheese evenly over the top. Bake for 15-20 minutes, or until the cheese is melted and bubbly and the edges are golden.