"Rustic rib-eye on ciabatta is grilled steak, pesto-lime aioli, sugar caramelized onions, melted cheese, and garlic butter artisan bread! Layer spinach, tomato, banana peppers, or any other fixings you desire with the steak."
Coat steaks with 1 tablespoon butter each; season with salt and pepper. Place each steak in a vaccum-seal bag. Seal the bags and place in a water bath with a sous vide cooker inserted. Set temperature to 130 degrees F (54 degrees C); cook until firm and reddish-pink and juicy in the center, about 1 hour.
Combine 1 tablespoon butter, garlic oil, and brown sugar in a large nonstick skillet over medium heat until butter is melted, about 1 minute. Place onion into the pan in a single layer; cook until translucent, about 5 minutes. Add vinegar; cover and reduce heat to low. Cook, adding a splash of water if needed, until onions are golden brown, about 20 minutes.
Combine pesto sauce, mayonnaise, and lime juice in a bowl. Stir until well combined. Cover with plastic wrap and refrigerate until ready to use.
Preheat oven to 325 degrees F (165 degrees C).
Stir melted butter, onion powder, garlic powder, and salt together in a bowl. Brush mixture over both halves of the bread. Place bread on a baking sheet; add Muenster cheese to 1 piece.
Bake in the preheated oven until golden brown, about 15 minutes.
Heat a grill pan over medium-high heat. Remove 1 steak from the bag and cook on the grill pan until nicely browned, about 1 minute per side. Repeat with the remaining steaks. Let rest until set, about 5 minutes. Slice thinly against the grain.
Spread pesto mixture over the bread; add the steak and browned onions. Cut sandwich into 6 pieces.