Mix together butter and rosemary. Spread on the four slices of bread. 

In a food processor, combine feta and lemon juice. Add olive oil and pulse until the mixture is smooth and spreadable. 

Heat a skillet (preferably cast iron) over a low heat. Spread feta on the non-buttered side of each slice of bread. 

Lay two slices of bread butter-side down on the heated skillet. Layer one slice of provolone, half the mozzarella, and half the gruyere on the slices of bread. Cook open-faced until cheese begins to melt. Flip one slice of bread onto the other and cook for an additional minute or so. 

Repeat with the other sandwich. Cut and half and serve immediately with tomato soup or on its own.