Grilled Beef and Marinated Fennel Sandwiches with Mustard Sauce | RICARDO

Ingredients

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