In a large skillet, melt the butter over medium heat. Add the chopped onions and sauté for 15-20 minutes, stirring occasionally, until golden brown and caramelized.
In a mixing bowl, combine the lean ground beef and ground sirloin. Pour in the Cabernet Sauvignon, then add celery salt, dry mustard, black pepper, salt, garlic powder, and dried thyme. Gently mix until just combined.
Divide the meat mixture into eight equal portions and shape them into patties, making a slight indentation in the center of each.
Preheat your grill or skillet over medium-high heat. Cook the patties for 4-5 minutes on each side for medium-rare, or until they reach desired doneness (160°F internal temperature recommended).
Toast the hamburger buns. Place a burger patty on the bottom half of each bun, top generously with sautéed onions, then finish with the top bun. Add additional toppings if desired.