Once the dough has chilled, start layering the fillings. First, start with the potatoes. Create a layer using half of the potato slices. On top of the potatoes, spread half of the melted cheese mixture. Repeat the same two layers, using up the remaining potatoes and cheese. Top the last layer of cheese with the roasted fennel. Top the tart with the reserved Gruyere and pecorino cheeses and sprinkle with fresh rosemary. Bake for 30 minutes or until golden-brown and sizzling hot. Let cool slightly (or completely) before serving.
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