Heat a large non-stick sauté pan over medium heat. Add the walnuts and shake the pan continuously until toasted and fragrant, about 6 minutes. Remove to a bowl, let cool, then coarsely chop.
Return nonstick sauté pan to medium-high heat. Brush the cheese with olive oil and cook, turning once, until lightly browned in spots, about 5 minutes. Transfer to platter and let cool.
In a large mixing bowl add corn, scallions, parsley, mint, walnuts, grain mixture and arugula.
Using your hands, tear the halloumi into bite-size pieces; add to bowl and toss to combine.
Add the dressing to the salad, two tablespoons at a time, as desired and toss to coat. Add the watermelon and toss again to combine.