Prep the citrus: Using a sharp knife, slice off the top and bottom of the orange and grapefruit. Stand each fruit on the flat end and cut away the peel and pith, following the curve.
Cut out segments between the membranes, catching juices in a bowl. If you prefer, slice into thin rounds instead of segments.
Slice the fennel: Trim off stalks and fronds, reserving some fronds. Halve the bulb vertically and remove the core.
Slice as thinly as you can with a knife or mandoline. Place in a large bowl.
Make the dressing: In a jar or small bowl, whisk together 3 tablespoons olive oil, 2 to 3 tablespoons lemon juice (to taste), lemon zest, 1 teaspoon Dijon, and 1 to 2 teaspoons honey. Season with a generous pinch of salt and several grinds of pepper.
Whisk until emulsified.
Toss the base: Add arugula and mint to the bowl with fennel. Drizzle on about half the dressing and toss gently until lightly coated. Taste and adjust salt and pepper.
Add the citrus: Gently fold in the orange and grapefruit segments along with any reserved juices.
If the salad needs more brightness, add a spoonful more dressing.
Finish and garnish: Top with torn fennel fronds and any optional add-ins like toasted nuts or shaved cheese. Give it one last light toss so the citrus stays mostly intact.
Serve right away: This salad shines when it’s crisp and fresh. Plate it on a wide platter so the colors show off.