In a large sauté pan, melt butter over medium heat. Add the onions and garlic and sauté until tender, about 5 minutes.
Add white wine and mussels. Cover and bring simmer for 5 minutes, or until mussel shells begin to open. Once mussel shells begin to open, add Mediterranean Grille sauce and toss.
Continue to cook uncovered for an additional 5 minutes or until the majority of mussels are opened. Toss with parsley and capers until sauce is well dispersed. Sprinkle with feta cheese crumbles before serving.