For the marinade: In a medium bowl, whisk together the oil, lemon zest, lemon juice, garlic, cumin, salt and pepper flakes until smooth. Add the chicken and toss until coated with the marinade. Cover and refrigerate for at least 4 hours or overnight. (Can also be made in a resealable plastic bag.)
For the pesto: In a food processor, blend the mint, spinach, cheese, walnuts, garlic, lemon juice, salt and pepper until chunky. With the machine running, slowly add the olive oil until smooth.
Place a grill pan over medium-high heat or preheat a gas or charcoal grill. Spray the grill lightly with vegetable oil cooking spray. Remove the chicken from the marinade. Discard the marinade. Grill the chicken until cooked through, 4 to 5 minutes on each side. Place on a platter and serve with the pesto.
To toast the walnuts, arrange in a single layer on a baking sheet. Bake in a preheated 350 degrees F oven until lightly toasted, 5 to 8 minutes. Cool completely before using.