Pat the ribeye completely dry with paper towels. Season generously with salt and pepper on both sides. Let sit at room temperature for 15 minutes while you prep other ingredients.

Heat olive oil in a large oven-safe skillet over medium-high heat until shimmering. Sear the steak for 3-4 minutes per side until deeply browned. Transfer to a plate to rest. The center should still be rare - it will finish cooking in the oven. Do not clean the pan.

Reduce heat to medium. Add the sliced onion to the same skillet with the steak drippings. Cook for 12-15 minutes, stirring occasionally, until deep golden and softened. If the pan looks dry, add a splash of water to deglaze. Add garlic and cook 1 minute until fragrant.

Pour in heavy cream and Worcestershire sauce. Simmer for 3-4 minutes until slightly thickened. Stir in half the blue cheese (3 oz), Parmesan, and thyme. Season with salt and pepper - the blue cheese is salty, so taste first.

While the sauce simmers, boil rigatoni in salted water for 2 minutes less than package directions - it should still have a firm bite. Reserve 1 cup pasta water, then drain.

Slice the rested steak against the grain into thin strips, then cut into bite-sized pieces - you will see pink in the center, which is perfect. Add pasta and steak to the skillet with the sauce, tossing to coat. If it looks tight, loosen with reserved pasta water, 1/4 cup at a time.

Preheat broiler. Scatter remaining blue cheese over the top. Broil 4-6 inches from heat for 3-5 minutes until the cheese is melted and spots are golden brown. Let rest 5 minutes before serving - the sauce will thicken as it sits.