Roast the Peppers : Preheat your oven's broiler to high. Place the whole red bell peppers on a baking sheet and broil for 10-15 minutes, turning every few minutes with tongs, until the skin is blackened and blistered on all sides. Transfer the peppers to a bowl and cover tightly with plastic wrap to steam for 10 minutes. Once cool enough to handle, peel away the charred skin, remove the stem and seeds, and chop the roasted flesh into a medium dice.
Assemble the Base : In a large glass mixing bowl, combine the rinsed and drained cannellini beans, diced English cucumber, and finely chopped red onion.
Add Aromatic Elements : Add the rinsed and chopped Kalamata olives, the rehydrated and chopped sun-dried tomatoes, and the rinsed capers to the bowl. This step adds layers of savory, briny flavor.
Prepare the Vinaigrette : In a small jar with a lid, combine all vinaigrette ingredients: the chopped parsley, basil, oregano, extra-virgin olive oil, lemon zest and juice, minced garlic, Dijon mustard, and black pepper. Seal the jar and shake vigorously for 30 seconds until the dressing is emulsified and slightly creamy.
Combine the Salad : Add the chopped roasted red peppers to the salad base. Flake the drained tuna into large chunks and gently add it to the bowl, being careful not to break it up too much.
Dress and Marinate : Pour the prepared vinaigrette over the salad ingredients. Using a rubber spatula, gently fold everything together until just combined. For the best flavor, cover and refrigerate for at least 30 minutes before serving to allow the ingredients to meld.