Heat olive oil in a large pot over medium-high heat. Add chopped onions. Cook for 5 minutes. Add garlic and tomatoes. Cook for an additional 5 minutes.
Add liquid, herbs and seasonings. Bring to a moderate boil for 10 minutes.
Finally add cooked lobster meat, shrimp, mussels and flounder (or other white meat fish). Stir gently. Cover and bring to a boil for an additional 5-10 minutes. The mussels are done when they have opened. If some do not open, discard. The shrimp will turn an opaque color with a hue of pink and the flounder will be flaky and white.
Remove from heat. Serve warm with fresh bread and Parmesan cheese, if desired.