Eye fillet with pine nut, feta and spinach stuffing

Ingredients

• 1.25 kilogram piece Quality Mark eye fillet of beef
• 30 gram butter
• 1 onion, peeled and diced
• 1/4 cup pine nuts
• 1 tablespoon fresh chopped sage leaves
• 100 gram baby spinach leaves, stems trimmed
• 1 1/2 cup fresh breadcrumbs
• 1 egg, beaten
• 60 gram feta cheese
• 1 tablespoon olive oil
• salt and freshly ground black pepper to taste