Toss the shrimp, scallops and squid with the salt. Heat a pan over medium heat and add some oil to lightly coat the bottom. Sear the seafood briefly in a single layer in batches until both sides are lightly browned (no need to cook through). It takes about 20-30 seconds each side, depending on your stove. Set aside.
In the same pan, also sear the mushrooms until both sides are lightly browned. Set aside.
To the same pan, add some more oil if needed. Then add sliced onions and minced garlic, sauté for a few seconds until garlic is fragrant. Then add carrot, stir and cook for up to another minute.
Add shaoxing wine to the pan, then add oyster sauce and water (reserving 1 tablespoon of water). Stir and let the mixture come to a simmer.
Dissolve cornstarch in the reserved water, and slowly pour the cornstarch slurry into the sauce while stirring to thicken it. It should be thick enough to coat the back of a spoon. Turn off the heat and set the sauce aside.
Bring a large pot with plenty of water to a boil. Add snow peas and blanch for several seconds so that they are still crisp and look vibrant green. Remove the snow peas and set aside.
To the same pot, add tofu cubes and also blanch briefly for several seconds. Remove from the water and set aside. Be careful not to break the tofu.
Transfer the sauce made in Step 5 to a clay pot or a small dutch oven. Then add half of the tofu and mushrooms. Continue to place seared seafood into the pot. Arrange the remaining tofu cubes, mushrooms and snow peas on top of the seafood.
Cover the clay pot (or dutch oven) and simmer over medium low heat for 4-5 minutes, or until everything is cooked through and the dish is bubbling. Open the lid and give the casserole a gentle stir.
Taste and adjust seasonings to your liking by adding more salt (or soy sauce) if needed. Sprinkle with some black pepper. Remove the pot from the heat and serve right away.