Get the lobster pot setup on the burner. Add about 5 inches of water and bring the pot to a boil over high heat.
White the water is heating, add the onions, garlic, bay leaf, sea salt and peppercorns, to the center of a large square of cheesecloth. Gather the four corners to form a pouch, and tie with kitchen twine. Drop the pouch into the boiling water.
While the seasoning is infusing, gather the cleaned clams, steamers, and mussels loosely into large squares of cheesecloth, and tie the corners to form pouches. Set aside.
Add the potatoes to the steamer basket, and scatter about half the herbs on top of the potatoes (if you have seaweed from the market, include some here as well).
Insert the steamer basket into the pot. The potatoes should be submerged in the water. If not, add more water until the potatoes are covered. Cover and boil the potatoes for about 5 minutes.
Once the potatoes are partially cooked, add the corn and remaining herbs to the steamer. Cover and cook for another 5 minutes.
Snip off the elastic bands from the claws, and add the lobsters to the pot.
Layer the clams, steamers, and mussel pouches on top, cover and steam until the lobsters are bright red, and the clams and mussels open, about 7 to 9 minutes for 1 1/4-pound lobsters (see notes for other sizes).
To check, stick an instant read thermometer into the area where the tail meets the body. Lobster meat is fully cooked when it reaches 140ºF. If you don't have a thermometer, cut the shell where the tail meets the body. If the meat is white, the lobster is fully cooked. If it is still translucent, return it to the pot.
Turn off the heat, uncover the pot, and pull out the pouches of clams, steamers, and mussels. Open the pouches and dump into serving bowls. Discard any that have not opened.
Remove the lobsters using kitchen tongs. Snip the tips of each claw with kitchen shears and hold the lobster upside down to let the liquid drain back into the pot. Place the lobster into serving bowls.
Lift the steaming basket, let the broth drain back into the pot, and transfer corn and potatoes to serving bowls.
Ladle broth over the lobsters and clams, and into several small bowls for rinsing the meat after extracting from the shells.
Serve warm with lemon wedges, melted butter, and the rest of your side dishes. Serves 6 people.