Wash and halve the tomatoes. Spinach wash, sort and drain. Peel and chop the onion. Grate the feta roughly. Whisk eggs and milk, season with salt, pepper and nutmeg.
Heat 2 tablespoons of oil in an ovenproof pan (24 cm Ø). Brown the onions in it. Add the tomatoes and spinach, fry briefly until the spinach collapses. Pour egg milk on top. Sprinkle with feta.
Cook in the preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer) for about 10 minutes until the egg has set.
In the meantime, wash and sort the salad and drain well. Whisk vinegar with salt, pepper and sugar. Fold in 4 tablespoons of oil in a thin stream. Mix salad with vinaigrette, season to taste. Mix crème fraîche with mustard, season to taste with salt and pepper.
Carefully remove the omelette from the edge of the pan and slide it out of the pan onto a plate. Quarter the omelette, arrange quarters on plates with salad and mustard cream. Dust mustard cream with a little nutmeg if necessary.