Preheat the oven to 180°C conventional oven/ 160°C fan-forced (350° Fahrenheit/Gas 4). Grease a round baking dish with butter (alternatively, line a baking dish with baking paper).
To make the Spinach and Potato mixture for the frittata, melt the butter and olive oil in a medium-sized frying pan over medium heat. Add the onion and potatoes. Cook, continuously stirring until the potatoes are golden and tender (approximately 8 minutes though will vary depending on the thickness of the potatoes). Add the spinach and cook until the spinach wilts. Set aside to cool slightly.
Use a fork to lightly whisk the eggs in a bowl until combined. Stir in the Parmigiano Reggiano (or Grana Padano), rosemary and season well with salt and pepper to taste.
Arrange the spinach and potato frittata mixture on the base of the greased baking dish. Scatter the Provolone cheese over the spinach and potato and pour the egg mixture evenly over. Bake the frittata for 25 minutes.
Allow the Baked frittata with spinach and potatoes to cool for 5 minutes before serving – buon appetito.