Dice both bell peppers into uniform 1/2-inch pieces, dice the cucumber into 1/2-inch pieces, and thinly slice the red onion. Finely chop the parsley and thinly slice the fresh mint and basil leaves. Having all your vegetables and herbs prepped and ready will make assembly quick and ensure even distribution of flavors throughout the salad. Set everything aside in separate bowls or on a cutting board.

Mince the garlic cloves finely and combine them with the Dijon mustard, lemon juice, sugar, and dried oregano in a large bowl. Whisk these ingredients together to combine, then slowly drizzle in the olive oil while whisking constantly to emulsify the vinaigrette. Season with salt and black pepper to taste. I like to taste the dressing at this point and adjust the lemon or salt—a well-balanced vinaigrette is the foundation of the entire salad, so don't rush this step.

Rinse and drain all three cans of beans (kidney beans, cannellini beans, and chickpeas) thoroughly under cold water to remove excess sodium and starch. Add the drained beans directly to the vinaigrette in the large bowl and stir well to coat each bean with the dressing. The beans will absorb the flavors more effectively while still warm and wet, so don't skip this step of coating them before adding the vegetables.

Add all the prepped vegetables (bell peppers, cucumber, red onion), the capers, and the fresh herbs (parsley, mint, and basil) to the bean mixture. Stir gently but thoroughly to distribute all components evenly throughout the salad, ensuring each spoonful will have a balanced mix of beans, vegetables, and fresh flavors. I find that a gentle hand with fresh herbs helps preserve their texture—avoid crushing them as you fold everything together.

Cover the salad and refrigerate for at least 30 minutes to allow the flavors to meld and develop. After chilling, give the salad a gentle toss and taste again, adjusting salt and pepper as needed since chilling can mute flavors slightly. Serve chilled or at room temperature, depending on your preference.