Brush sausages and onion slices with olive oil and place on the grill. Cook until browned on all sides.
Put the sausage and onion into a saute pan, add the wine and cook until the wine is completely reduced. While the wine is reducing, break up the sausage into small pieces. Add the spinach, 1/2 cup of water and season with salt and pepper. Cook until the spinach has wilted, about 2 minutes. Remove from the heat and stir in the 2 tablespoons of the basil, 2 tablespoons of the parsley and the cheese and let cool slightly.
Heat the grill to high.
Brush the mushroom caps on both sides with the remaining oil and season with salt and pepper. Place the mushrooms on the grill, top side down and cook until golden brown, about 4 minutes. Remove the mushrooms from the grill and place on a flat surface, cap side down. Fill the mushrooms with the sausage mixture and top each with a few slices of tomato. Season the tomatoes with salt and pepper and place on the grill, cap side down, close the cover and cook until the mushroom and filling have heated through, the cheese has melted and the tomatoes are soft, about 5 minutes. Remove to a platter and sprinkle tops with the remaining parsley and basil.