Slice the onion. Peel the sweet potatoes, then coarsely grate or slice them.
Cut off the broccoli rosettes and cut them into small pieces. Mix the eggs with 3 Tbsp of water and 6 pinches of salt.
Heat the oil in a large pan and soften the onions. Add the sweet potatoes and sauté for 5 minutes. Add the garlic and thyme leaves, then stir for one minute.
Meanwhile, steam the broccoli above boiling water or blanche it for a couple minutes in boiling water.
Drain and add to the sautéd sweet potato & onion mixture.
Crumble the feta onto the mixture then pour over the eggs.
Cook for 15 minutes on a low heat.
Turn on the grill in the oven. When the top of the frittata is jelly-like brown it under the grill for 2-3 minutes in the oven.
Serve warm or at room temperature.