Whisk together vinegar and mustard in a large mixing bowl until smoothly combined.
Slowly drizzle in olive oil while continuing to whisk, creating a creamy vinaigrette.
Add 1/2 teaspoon salt and a few grinds of black pepper, then adjust seasoning to taste.
In the same bowl, toss in the cannellini beans, kidney beans, chickpeas, tuna, fennel, celery, carrot, and kale. Mix gently but thoroughly to coat everything evenly with the dressing.
Sprinkle parsley, mint, and chopped fennel fronds over the salad, and toss lightly again to combine.
Taste once more. Add more salt or pepper if necessary, then serve immediately!
If meal-prepping, cover and store in the refrigerator for up to 3 days. This salad’s flavors intensify beautifully over time.