Peel and slice the sweet potatoes (¼ inch thick). Place the slices in an oven dish and coat them with a mix of olive oil, cumin and a pinch salt. Roast them for 20 minutes in the oven at 180°C (350°F).
Line the cups of a muffin pan with parchment paper. In the cups, spread the sweet potatoes, spinach and parmesan.
In a large mixing bowl, beat the eggs with the creme fraiche. Pour the mix into the cups. Bake for 25 minutes or so. Let cool for 5 minutes before removing the frittatas from the cups.
