1. In a shallow dish, combine the balsamic vinegar, about 2 tablespoons of the olive oil, and the grill seasoning. Coat the chicken breasts in the mixture and set aside to marinate while you start the white beans and spinach.
2. Heat a large skillet over medium high heat with the remaining 2 tablespoons of olive oil; Add the onions, garlic, thyme, red pepper flakes, salt, pepper and bay leaf. Cook, stirring frequently, until the onions are a little brown, 3-4 minutes. Add the white wine and chicken stock, bring up to a sbuble, and cook for 5 minutes.
3. Heat another large skillet over medium high heat. When it is hot, add the chicken breasts and cook for 5 to 6 minutes on each side. Remove the chicken to plate, cover loosely with foil, and let rest a few minutes.
4. Add the cannellini beans to the skillet with the onions and stir to combine. Cook for about 2 minutes or until the beans are heated through. Turn off the heat and discard the bay leaf. Stir in the spinach, parsley, and lemon juice. Toss and stir until the spinach wilts.
5. To serve, place a portion of the white beans and spinach on each serving plate. Slice each chicken breast and arrange over the beans and spinach.