After zesting the grapefruit and oranges, slice the peel and pith off, cut fruit into slices and reserve.
Preheat a small skillet over medium heat to toast the slivered almonds.
Preheat a large skillet over medium-high heat with a few turns of the pan of EVOO, about 3 tablespoons. Season the fish filets with salt and pepper, and transfer to the hot skillet. Cook for 4-5 minutes on each side or until cooked to desired doneness.
Meanwhile, preheat another large skillet with two turns of the pan of EVOO, about 2 tablespoons, over medium heat. Add the red onion, the orange and grapefruit zest, mustard and white wine vinegar. Season with some salt and pepper. Just before you are ready to plate, add in the spinach and toss.