Scrub and dice the potatoes into 2cm cubes. Put in a pan of cold water. Bring to the boil and simmer for 6-8 mins until tender. Drain and set aside.
Meanwhile, thinly shred the chard leaves and stalks. Peel and grate the garlic. Heat 2 tsp olive oil in a frying pan. Add the chard and garlic to pan and cook, stirring, for 2-3 mins over a high heat to wilt the chard. Take off the heat.
Zest half the lemon into a bowl. Set aside. Squeeze a little juice over the chard, then tip the chard mixture onto a large plate to cool a little.
Pick the feathery leaves from the dill and roughly chop. Place into a large bowl with the eggs, saffron powder, 1 tbsp of the crème fraîche and plenty of salt and pepper. Whisk together.
Stir the remaining crème fraîche into the bowl with the lemon zest. Finely chop the tarragon leaves. Add them to the bowl. Season with a little salt and pepper. Stir together. Set aside.
Heat a small non-stick frying pan and add 1/2 tbsp oil. Add half the egg mix and cook gently for 2-3 mins, tilting the pan to cover the base of the pan. When the omelette is just firm and the top is almost set, top with a spoonful of the drained potatoes and the chard. Fold the omelette in half. Cook for 1-2 mins then slide out onto a hot plate.
Repeat with another 1/2 tbsp oil, the remaining egg mixture and a spoonful each of the potatoes and chard to make a second omelette. Slide onto a warm plate.
Serve the omelettes with the remaining potatoes and chard on the side and spoonfuls of the tarragon crème fraîche.