Carbonade Flamande Stew

Ingredients

• • 1 kg (2 lb) flank steak, cut into 4 pieces
  • 30 ml (2 tbsp) olive oil
  • 4 large onions, thinly sliced
  • 30 ml (2 tbsp) butter
  • 30 ml (2 tbsp) all-purpose flour
  • 1 bottle 341 ml red or dark beer
  • 125 ml (1/2 cup) chicken broth
  • 30 ml (2 tbsp) brown sugar
  • 30 ml (2 tbsp) balsamic vinegar
  • 15 ml (1 tbsp) Dijon mustard
  • Salt and pepper