In a small ovenproof frying pan, heat olive oil over medium heat. Add the sausage, vegetables and spring onion. Cook until the sausage is heated through and the vegetables are tender.
Season the beaten eggs with salt and pepper, then pour into the frying pan. Stir briefly, then leave to allow the bottom of the frittata to set. Once you see the bottom setting, take a spatula to the edges of the pan to ensure that the frittata isn't sticking.
When the frittata starts to set around the edges, sprinkle the mint and chilli flakes over the top. Place the pan under the grill for 5 to 10 minutes, until the frittata is fully set and the edges slightly browned. Serve immediately or at room temperature.
I don't find cheese necessary in this frittata thanks to all the flavour from the merguez, but you could opt to add about 50 to 75g crumbled feta cheese or a milder soft goat's cheese. For even more spice, whisk a bit of harissa into the beaten eggs!