Place onions, garlic, red pepper flakes, beer, and potatoes in the bottom of a large stock pot.
Layer the clams over the potato mixture.
Add the corn on top of the clams.
Place the lobsters on the very top layer.
Cover tightly with a lid and cook on medium-high heat for 15-20 minutes.
Check that potatoes are soft when pierced with a fork, clams have opened, and lobsters are bright red.
Serve immediately with lemon wedges and melted butter.