Beef and Caramelized Onion Toasts

Ingredients

• • ½ cup (125 ml) sour cream
  • 1 tbsp (15 ml) whole-grain mustard
  • 1 tbsp dill, finely chopped
  • 1 tsp (5 ml) prepared horseradish (optional)
  • 1 lb (450 g) beef top sirloin steak, ¾ inch (2 cm) thick
  • 2 tbsp (30 ml) olive oil
  • 2 small red onions, thinly sliced
  • ¼ cup (60 ml) water
  • 4 slices country bread, each ¾ inch (2 cm) thick, toasted
  • ¾ cup (20 g) arugula
  • Sharp cheddar cheese shavings, to taste (see note)