Directions

1. Put red onion in a small bowl of ice water and set aside for 10 minutes to soften its harsh flavor while you prepare the rest of the salad.
2. Cut off fennel stalks (the top part that looks like celery, if present) from the bulb. Separate feathery green fronds from stalks and chop them finely to make 1-2 Tbsp. Discard remaining fennel stalks and fronds or save for making stock. Cut out the hard V-shaped core from the bottom of the fennel bulb. Slice fennel crosswise about 1/8 inch thick to make about 1 cup.
3. Put sliced fennel and chopped fronds in a large serving bowl. Seed and thinly slice red bell pepper and add it to the bowl. Drain onion and add it to the bowl along with beans and tuna.
4. Put olive oil, lemon juice, mustard, garlic, salt, and pepper in a small jar with a lid or in a small bowl and and shake vigorously or whisk to combine. Pour dressing over the salad and toss gently to combine. Serve immediately, or store in an airtight container in the refrigerator up to 24 hours.