Preheat grill to medium high.
Brush mushrooms with olive oil. Season the bells of the mushrooms with salt and pepper.
Into the upside down bell of the mushroom, divide thawed spinach that has been squeezed of all additional moisture.
Top with chopped tomatoes, and top with crumbled feta cheese.
Top each mushroom with a sprig of rosemary.
Place on the grill and grill until the cheese has started to melt slightly and turn golden grown and the mushroom has grill marks on it and is fork tender. Remove and serve immediately.
