Flat Iron Sandwiches with French Onion Au Jus


Flat-Iron Sandwiches with French Onion Au Jus

Flat Iron Sandwiches with French Onion Au Jus

This recipe is for the serious sandwich lover. The steaks are marinated, then grilled to perfection, sliced and layered onto bread with onions from the au jus, Swiss cheese, and a mustard-black pepper mayonnaise. Dip this heaping sandwich into piping hot au jus for the greatest food experience
Prep Time 35 mins
Cook Time 1 hr 5 mins
Total Time 1 hr 40 mins
Course Main Dish
Cuisine American BBQ
Servings 12 People


  • 4 pounds flat-iron steaks
  • for marinade:
  • 1/3 cup olive oil
  • 1/4 cup cider vinegar
  • 2 tablespoons sea salt
  • 2 teaspoons herbes d' provence
  • 2 teaspoons Worcestershire sauce
  • 1 teaspoon cumin
  • 1 teaspoon paprika
  • 1 teaspoon onion powder
  • 1/2 teaspoon garlic powder
  • 1/2 teaspoon chilli powder
  • 1/2 teaspoon coarse ground black pepper
  • For Au Jus
  • 21 tablespoon beef baste, plus 2 cups water
  • 1 bottle ale
  • 2 medium onions, thinly sliced into rings
  • 2 teaspoons sea salt
  • 1 teaspoon Worcestershire sauce
  • 1/4 teaspoon black pepper
  • dash of chili powder
  • For Mayonnaise:
  • 1/2 cup mayonnaise
  • 2 tablespoons prepared mustard
  • 1/2 teaspoon coarse ground black pepper
  • 12 crunchy deli rolls


  • Place flat-iron steaks in a resealable plastic bag. Combine marinade ingredients and pour over steaks. Make sure every bit of the steaks are well coated. Seal bag and place into refrigerator. Allow meat to marinate for 2-4 hours before grilling.
  • While steaks are marinating, prepare mustard-black pepper mayonnaise by combining mayonnaise, mustard and black pepper in a small bowl. Place into an airtight container. Refrigerate until ready to use.
  • To prepare au jus, combine beef base with 2 cups warm water. Add to a large saucepan. Add ale, sliced onion, and remaining ingredients for jus. Allow mixture to come to almost a boil, reduce heat and simmer for 35-45 minutes. If sauce begins to reduce too much, simply add 1/2 to 1 cup of water until it reaches the right consistency.
  • Preheat grill for medium-high heat. Remove steaks from bag and place onto grill. Cook for 8-10 minutes per side, depending on thickness. You can reduce or increase cook times if needed. Once steaks have cooked to desired doneness, remove from grill, tent with foil and allow them to rest for 5-7 minutes before carving. Slice against the grain.
  • To assemble, apply mayonnaise mixture first, then two slices of Swiss cheese per sandwich. Next, place desired amount of simmered onions from au jus, onto bread. Finally add steak slices. Using a ladle, scoop (or strain) au jus , minus onion into a small bowl. Dip sandwich into jus and enjoy.