Melt the butter in a large skillet over medium heat. Cook and stir the onion in the melted butter until softened, about 5 minutes. Stir the ketchup, lemon juice, Worcestershire sauce, brown sugar, and yellow mustard into the onion and butter; allow mixture to warm completely.
Heat a separate skillet over medium-high heat. Season the steaks with salt and black pepper; cook in the skillet until browned on each side, 3 to 4 minutes per side. Transfer the steaks to a 9x13-inch baking dish. Pour the sauce from the skillet over the steaks.
Bake in the preheated oven until meat is cooked through and no longer pink in the middle, about 1 hour.
Transfer the steaks from the baking dish to a platter, shaking any excess sauce from the meat. Stir the cornstarch and water together until smooth in a small bowl, then mix into the hot sauce remaining in the baking dish until the mixture begins to thicken; pour over the steaks to serve.