In a medium bowl, beat the 4 oz softened cream cheese and 3/4 cup mayonnaise together until smooth and well combined.
Stir in 1/2 tsp garlic powder, 1/8 tsp seasoned salt, 1 cup shredded cheddar, and 1 cup shredded mozzarella until the mixture is evenly combined.
Cut the French bread loaf into 1/2" thick slices and pair slices to form sandwiches.
Spread the cheese mixture evenly over one slice of each pair. Top with the second slice so the spread sides are facing each other (no butter needed).
Preheat a nonstick or well‑seasoned skillet over medium heat. Place the assembled sandwich in the skillet with the plain outer sides down. Cook until the bottom side is golden brown, about 3–5 minutes.
Flip the sandwich and continue cooking until the second side is golden brown and the cheese is fully melted, about 3–5 more minutes; if needed, cover the skillet briefly to help the cheese melt.
Transfer to a cutting board, let rest 1 minute, then slice and serve.