In a large skillet, heat oil.  Cook garlic and onions until soft (about 3 minutes).  Add spinach and cook until wilted and soft.  Pour out any liquid that accumulates in pan.  Add feta cheese and sundried tomatoes and mix well to combine.

Preheat oven to 400 degrees.  Grease a baking dish.

Place fillets, smooth side up, on a work surface.  Sprinkle with salt, pepper and lemon juice.  Divide the filling among fillets.  Spread the filling into an even layer.  Starting with the wide end, roll up to enclose filling.  Place seam side down on baking pan.  Brush with melted butter and sprinkle with capers.  Place lemon slices over fish.  Bake until cooked to the center, about 25 minutes.