Heat oven to 400°F. Lightly grease a 9×13-inch baking dish if not using an oven-safe skillet.
Warm olive oil and 3 tbsp butter in a large skillet over medium-low heat.
Add onions and 1 tsp salt. Cook slowly, stirring often, until deep golden, 25–30 minutes.
Stir in thyme, garlic, and pepper. Cook until fragrant, about 1 minute.
Pour in sherry and Worcestershire. Scrape up browned bits and simmer 2 minutes to reduce.
Stir in dry orzo to coat with the onions. Add beef broth and bring to a gentle simmer.
Transfer mixture to the baking dish if needed. Cover tightly with foil and bake 15 minutes.
Uncover, stir, and top evenly with Gruyère and Parmesan.
Combine breadcrumbs with melted butter. Sprinkle over the cheese.
Bake until orzo is tender and the top is bubbly and golden, 10–12 minutes. Broil 1–2 minutes if needed.
Rest 5 minutes. Sprinkle with parsley and serve warm.