Place chicken in a glass or ceramic dish. Combine lemon juice, paprika and oregano in a jug. Pour over chicken. Cover with plastic wrap and place in fridge for 30 minutes to marinate.
Heat oil in a large frying pan over medium heat. Cook onion, stirring, for 5 minutes or until soft. Add the spinach and garlic and cook, stirring, for 1 minute or until the spinach just wilts. Set aside for 5 minutes to cool slightly. Transfer to a food processor.
Add basil, feta, pecan, and pecorino to the spinach mixture. Process until finely chopped. Add evaporated milk and extra oil. Process until combined. Season with salt and pepper.
Cook spaghetti in a large saucepan of salted boiling water until al dente. Drain and return to the pan. Add the spinach pesto and toss to combine.
Meanwhile, preheat a chargrill on medium-high. Cook the chicken for 4 minutes each side or until cooked through. Slice the chicken across the grain.
Divide the pasta mixture and chicken among serving plates.