Prepare Zucchini Agrodolce; cover and keep warm.
Sauté onion in 2 tablespoons olive oil in a large skillet. Add garlic, and sauté 1 minute. Remove skillet from heat; stir in 1/4 teaspoon salt, 1/4 teaspoon pepper, 1/2 cup breadcrumbs, parsley, and next 4 ingredients.
Cut a 1 1/4-inch-long horizontal slit through the thickest portion of each steak, cutting to, but not through, other side to form a pocket. Stuff cheese-and-herb mixture into pockets; sprinkle with remaining 1/4 teaspoon salt and pepper.
Dip tuna into egg; dredge in remaining 1/2 cup breadcrumbs.
Panfry in remaining 3 tablespoons hot oil in a large nonstick skillet 4 minutes on each side or to desired degree of doneness. Serve with Zucchini Agrodolce.
Tip: Use a sharp, thin blade to make pockets in steaks. The point of entry should be no more than 1 1/4 inch wide, but fan the knife blade back and forth to create a large pocket inside the steak, being careful not to cut through the sides.