Create the dressing by whisking together the 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 the remaining 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!
By supremeing the grapefruit, you remove all the bitter pith which makes the grapefruit taste sweeter.