In a frying pan set on medium heat, add 1 tbsp olive oil, and the chopped spinach. Increase the heat to high and sauté till all the moisture has evaporated, the spinach darkens and begins to stick to the pan. Transfer to a small bowl and set aside.
In the same pan, add 1 tbsp olive oil, and sauté the mushrooms and garlic until golden-brown and all the moisture has evaporated. Transfer to a small bowl and set aside.
Next, cook the bell peppers in 2 tbsps olive oil, until lightly blistered for the best flavour. Transfer to a small bowl and set aside.
Finally, caramelise the onions in 1 tbsp oil until translucent and lightly browned. Set aside.
Preheat the oven to 175 C and grease a 9" pie dish.
In a large mixing bowl, add 1 tbsp olive oil and crack the eggs in, whisking with a fork to beat and combine. Add the milk and sour cream, followed by the mustard and seasoning. Mix well.
Add the cooked veggies to the egg custard and mix. Pour into the prepared dish and bake for 40 to 45 minutes, until the top is set and doesn't wobble. For a firmer texture, bake for an additional 10 minutes.
While the quiche cools, prepare the potatoes. Heat the frying pan (the same one from earlier to minimize dish washing) on high heat with 3 tbsps olive oil. Add the potatoes and garlic and toss well until coated in the oil. Allow to cook for 8 to 10 minutes until the potatoes are soft and golden brown on the outside. Turn off the heat and add the salt, pepper and chilli flakes. Transfer to a bowl, garnish with fresh basil and serve hot with the sliced quiche.