1. Cook the onion in the oil in a large saucepan over medium heat until softened, about 2 minutes. Add the pepper flakes and the wine. Bring to a boil and continue to boil for 3 minutes. Add the garlic and squeeze the lemon juice into the boiling broth. Place the lemon halves in the liquid and turn down to a simmer.

2. Add the clams and stir briefly. Cover the pan and simmer for 4 minutes. Uncover the pan and use tongs to remove any opens clams. (Remove these to a large bowl and cover them to keep warm.) Continue cooking, covered, until all the clams have opened, removing them as soon as they have. Keep warm. Discard any clams that have not opened after 8 minutes of cooking. 

3. Add the mussels and stir briefly. Cover and cook for 2 minutes. Remove mussels as they open and keep them warm with the clams. Continue to cook for 2 more minutes, discarding any mussels that have not opened in that time. Strain the cooking broth through a double thickness of moistened paper towels or a dampened coffee filter set in a strainer.

4. For the kefir, simmer the mint leaves in 3/4 cup of the strained broth for 2 minutes. Let cool lightly and mix into the yogurt, along with the lemon juice and a sprinkling of salt and pepper, stirring until smooth. (Set aside some of the kefir for another use, as you'll probably have more than enough.)

5. Serve the clams and mussels on a large platter with the kefir in a bowl for dipping.