For the chicken, combine the olive oil, sea salt, black pepper, Italian seasoning.
Add to a large plastic bag with the chicken and marinate for up to 8 hours, I would recommend at least an hour.
Remove the chicken from the marinade and place on a grill over medium heat for 10-12 minutes per side.
While the chicken is grilling, prepare the spinach mushroom topping.
Add the olive oil to a large skillet over medium heat.
Add the onions and mushrooms, sauté 5-7 minutes, add the garlic, sauté for 1 minute, add the spinach and sprinkle the Italian seasoning, salt and pepper over the top.
Saute for an additional 3-4 minutes, then turn the heat off and set the skillet aside.
Once the chicken is grilled, slice, place on a plate and top with the mushroom spinach topping.