Create the dressing by whisking together the grapefruit vinegar and mustard. While whisking, slowly drizzle in 3 tablespoons of the olive oil. Season to taste with sea salt and fresh ground black pepper. Set aside.
Using a vegetable peeler or mandolin, cut the zucchini into ribbons. Toss the ribbons with 2 tablespoons olive oil and sea salt.
Grill or sauté the zucchini slices over medium-high until crisp-tender
Transfer the hot zucchini to a large serving platter along with the spinach and arugula. Drizzle the vegetables with enough dressing to taste.
Garnish salad with grapefruit, toasted pumpkin seeds, and feta cheese. Enjoy!