Makes one 9-inch tart
3 tbsp unsalted butter, at room temperature
2 medium white onions, thinly sliced
2 medium leeks, sliced lengthwise and thinly sliced (white part only)
1 tsp fresh thyme leaves, finely chopped
2 tsp fresh marjoram leaves, finely chopped
1 tbsp fresh Italian parsley, finely chopped
1/2 cup crème fraîche
1 recipe basic tart pastry, blind-baked for 10 minutes
Melt 1 tablespoon of the butter in a large pan over medium heat. When it foams, add 1/3 of the onions and leeks. Season well with salt and freshly ground black pepper. Cook until onions and leeks are caramelized and golden brown, about 10 minutes. Season again with salt and pepper. Remove from the pan and reserve in a mixing bowl. Cook remaining onions and leeks in two more batches, using 1 tablespoon of the butter per batch.
Combine herbs and crème fraîche with onion-leek mixture and check the seasoning. Add more salt and pepper as necessary, and let the mixture rest until it has cooled, about 5 minutes.
Fill prepared tart-shell with mixture and bake for twenty minutes at 400° F.