1.Preheat the oven to 165C fan-forced or 185C conventional.
2. In a saute pan over medium heat,cook the speck until it starts to brown. Add the butter,garlic and onion and gently cook for 15-20 minutes or until soft and translucent but not browning. Set aside to cool.
3.While the onions cook,whiz the flour,butter and salt in a food processor to a sandy crumb. Add the egg and sour cream and process until it starts to form a ball. Form into a round between two sheets of baking paper and chill for half an hour.
4.When you are ready to bake the tart,in a large bowl whisk together the eggs,sour cream and flour,add the gruyere and the garlic and onion mix,season lightly with salt and pepper and mix through.
5.Roll chilled pastry into a rough rectangle and place on a baking tray on baking paper. Pour mix on to the pastry (mix will be stiffish and malleable),leaving a border around the edge of three to four centimetres. Fold edges up,crimping corners together to make a free-form tart. Sprinkle with paprika. Bake 30-40 minutes or until golden. Cool to room temperature.
6. To serve,toss the mussels through the dill with a squeeze of lemon. Cut the tart into portions,top with the mussels,serve.
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