IF ANY CLAMS/MUSSELS ARE OPEN, TEST THEM FIRST!! Press shell together with thumb and index finger; hold for a second, and then release. OR, shock the shells in a bowl of cold water. If shells close, then they are safe to eat. If they remain open, TOSS THEM
Rinse clams and mussels, and drain in a colander
Melt 1 tablespoon butter in wide bottomed saute pan
Add onions, and saute over medium-low heat until onions are light golden brown. Remove onions and set aside.
Add remaining butter, plus avocado or olive oil. When butter has melted, add garlic and cook until fragrant, 2-3 minutes.
Add white wine and red pepper flakes. Bring to low boil
Add clams, cover, and steam until they just open. Remove clams with slotted spoon to reserve broth. Clams will probably not all open at the same time, so just keep an eye on them, and remove as they open.
Add a little more wine, if necessary. Add mussels, cover, and steam until just open. Mussels will open much more quickly than clams.
Turn heat to low, and add clams and onion to pan. Squeeze juice of 1/2 lemon over clams and mussels. For more heat, sprinkle with a little more red pepper flakes
Transfer clams, mussels and broth to serving dish. Garnish with parsley and lemon zest.