Mix the onion and vinegar in a small bowl, cover, and set aside for 30 minutes, stirring halfway through. Combine the fennel, green beans, and carrots in a large bowl. Drain the chickpeas and add them to the large bowl with the tomatoes and radicchio; gently toss. Drain the onion slices, reserving the vinegar. Whisk the reserved vinegar with the olive oil. Add the salt and pepper. Drain the tuna and flake it into large chunks in a medium bowl. Toss the tuna with half the onion slices, 2 tablespoons of the vinaigrette, and the parsley. Toss the remaining vinaigrette and onion with the chopped vegetables. Divide the vegetables among 4 serving bowls and top with the dressed tuna.