Preheat a char grill pan or barbecue grill on high. Spray sweet potao, chicken, and asparagus with cooking oil. Grill sweet potato for 3-4 minutes each side or until tender. Transfer to a plate. Grill the chicken and asparagus for 2-3 minutes each side, or until cooked. Transfer chicken to a plate, cover with foil; let rest for 5 minutes.
Meanwhile, melt butter in a small saucepan over low heat. Stir in juice, capers and garlic. Cook and stir for 1 minute or until hot. Remove from heat, stir in oregano.
To serve, arrange chicken, asparagus, sweet potao, and spinach on plates and top with caper sauce.