Spanish tortilla

Name: Spanish tortilla
Prep time: 30 min
Cook time: 50 min
Total time: 1 h 20 min
Cuisine: European, Mediterranean, Spanish
Ingredients: 1 large white onion, sliced; 4 tbsp olive oil; 25g butter; 400g waxy potatoes, peeled, quartered and finely sliced; 6 garlic cloves; 8 eggs, beaten; handful flat-leaf parsley, chopped, plus extra to serve; 1 baguette, sliced; 4 vine tomatoes, peeled and coarsely grated; drizzle of olive oil