Preheat a grill pan over medium-high heat and preheat the oven to 400°F.
Peel and cube the sweet potato into 1/2-inch pieces, then roast on a parchment-lined tray for 20 minutes until tender.
Season the chicken breast with half of the sea salt, black pepper, and dried oregano.
Grill the chicken for 6-8 minutes per side until charred and cooked through to an internal temperature of 165°F.
In a small bowl, whisk together the lemon juice, dijon mustard, olive oil, and the remaining salt, pepper, and oregano.
Thinly slice the red onion and halve the cherry tomatoes.
Place the arugula in a large bowl and top with the roasted sweet potatoes, tomatoes, and onions.
Slice the grilled chicken into strips, place on top of the salad, and drizzle with the zesty dressing.