Bison burgers with blue cheese

Sauce:

• 125 ml (1/2 cup) mayonnaise
• 30 ml (2 tbsp) HP sauce
• 1 garlic clove, minced
• 15 ml (1 tbsp) chives, thinly sliced

Patties:

• 600 g (1 1/4 lb) extra-lean ground bison
• 30 ml (2 tbsp) Dijon mustard
• 10 ml (2 tsp) dried tarragon
• Salt and freshly ground black pepper
• 4 slices of blue cheese, about 1 cm (3/8 in.) each

Garnish:

• 1 onion, peeled and cut into rings
• 30 ml (2 tbsp) all-purpose flour
• 30 ml (2 tbsp) vegetable oil
• 4 portobello mushroom caps, about the same size as the patties (remove the black part)
• 30 ml (2 tbsp) olive oil
• 4 sesame buns
• A few leaves of frisée lettuce
• 1 tomato, sliced