Grilled Beef and Marinated Fennel Sandwiches with Mustard Sauce

Ingredients

• Beef and Sauce

  • 1 beef rib-eye steak, about ¾ lb (340 g) and 1 inch (2.5 cm) thick
  • 1/3 cup (75 ml) sour cream
  • 2 tbsp tarragon leaves, finely chopped
  • 2 tbsp sour gherkins, chopped
  • 2 tbsp (30 ml) Dijon mustard
  • 1 tbsp (15 ml) whole-grain mustard

• Marinated Fennel

  • 1 fennel bulb, thinly sliced on a mandoline
  • 1/4 tsp salt
  • 1 tbsp (15 ml) white wine vinegar

• Sandwiches

  • 2 red onions, thinly sliced
  • 2 tbsp (30 ml) olive oil, plus more for brushing
  • 4 slices sourdough bread, each about 1/2 inch (1.5 cm) thick (see note)
  • Fennel fronds, finely chopped