Mama Marie's Moroccan Tuna Brik

INGREDIENTS

• 1 tablespoon butter
• 1 small onion, finely chopped
• 2 (5 ounce) cans tuna packed in oil, drained
• 2 tablespoons chopped fresh parsley
• 1 teaspoon capers, or to taste
• 2 tablespoons grated Parmesan cheese
• salt and pepper to taste
• 10 sheets phyllo dough
• 1 quart vegetable oil for frying
• 1 lemon, cut into wedges