For the dressing: Whisk the vinegar, rosemary, lemon juice, garlic and lemon zest in a medium bowl until blended. Gradually whisk in the oil, salt and pepper.
For the vegetables: Add enough cold water to a heavy large pot to reach a depth of 2 inches. Bring the water to a boil over medium-high heat and add the salt. Add the green beans, carrots and cauliflower. Cook until crisp-tender, 5 minutes. Using a slotted spoon, remove the vegetables and place in a bowl to cool.
For the seafood: Add the wine to the vegetable cooking water. Adjust the heat so the liquid is barely simmering. Add the cod and simmer until cooked through, about 5 minutes. Using a slotted spoon, remove the cod and set aside. Add the clams and shrimp to the pot. Cover and cook until all the clams have opened and the shrimp are cooked through and opaque in the center, 4 to 5 minutes. Discard any unopened clams. Remove the pot from the heat.
To assemble the salad: Arrange the carrots, green beans and cauliflower on a large platter. Arrange the shrimp, clams and cod on top. Drizzle with half of the dressing. Arrange the lemon wedges and beets around the edge of the platter
Serve the cappon magro with toasted bread and the remaining dressing alongside.
To cook beets, place them in a medium saucepan. Fill the saucepan with water until the beets are covered. Bring to a boil, then lower the heat to a simmer. Cover and simmer until the beets are cooked through, about 30 minutes. Peel the beets under cold running water and cut each into 6 or 8 wedges. Alternatively, place the beets in a large microwave-safe bowl. Add 1 cup of water. Cover with plastic wrap and cook on high power until tender, about 15 minutes.