In a bowl, season the onions with salt. Add flour and toss until well coated. Shake off excess flour. Deep-fry the onions until golden brown. Remove and drain on plate lined with paper towels. Set aside and keep warm.
Season steaks with salt and pepper. In a small bowl, combine the 2 mustards. Brush 1 side with mustard and lightly flour the same side. In a heavy saute pan, heat the peanut oil and 1 tablespoon of butter. Sear meat, flour side down first, for about 2 minutes or until golden brown. Turn over meat and saute for another 2 minutes. Meat should be medium-rare. Transfer to a plate and keep warm.
Deglaze pan with lemon juice or balsamic vinegar and stock. Reduce by half. Finish sauce with the remaining 2 tablespoons of butter and minced herbs.
To serve: Place each rib-eye steak on a warm plate, nappe with the sauce, and top with crispy onions.