Combine the lemon, parsley, garlic, oil and salt in a blender until mostly smooth.
Toss the chicken with the gremolata and marinate overnight or for at least 4 hours.
Preheat the oven to 350°F.
Remove the chicken from the marinade, discard marinade and season chicken with salt and pepper. Roast on a baking sheet until cooked through, about 22 minutes.
Toss the bell peppers with olive oil, salt and pepper on a baking sheet. Roast for 10-15 minutes, until softened.
Add all of the sauce ingredients except the oil to a blender. Purée until smooth. Drizzle in the oil with the motor running until emulsified. Season with salt and pepper.
Heat a large skillet over medium heat with the olive oil. Add the garlic and onions and cook, stirring, until softened. Add the kale and cook until wilted. Season with salt and pepper.
To serve, divide the chicken between plates. Top with roasted peppers, kale and onions and drizzle with parsley sauce. Serve with lemon on the side. een