Preheat a grill or a grill pan to medium-high. In a shallow dish, combine the balsamic vinegar, Worcestershire sauce, 3 tablespoons of the EVOO and the grill seasoning. Add the chicken thighs and turn to coat evenly with the marinade.
In a small saucepan, heat the dried mushrooms and chicken broth over medium-low heat.
In a large skillet, heat the remaining EVOO, 2 turns of the pan, over medium heat. Add the onion, season with salt and pepper and cook for a couple of minutes to soften. Add the rice, stir to coat and cook for 1 minute. Add half of the wine; when it has been absorbed by the rice, stir in the remaining wine and the rosemary. When all of the wine is absorbed, ladle in some of the mushroom broth.
Place the marinated chicken thighs on the grill and cook for 7 to 8 minutes on the first side and for 5 to 6 minutes on the reverse side.
Add a ladle of the mushroom broth to the risotto every few minutes, stirring occasionally and cooking until absorbed. Remove the softened mushrooms from the broth and chop; add to the risotto. Cook the risotto until creamy and al dente, 20 to 22 minutes. Remove from the heat; stir in the butter and cheese.
Thinly slice the grilled chicken. Divide the risotto among shallow bowls and top each serving with the arugula, spinach and sliced chicken.