Preheat the oven to 425°F. Line a heavy-duty baking sheet with aluminum foil.

Execute a longitudinal cut to halve the French bread horizontally.

In a small skillet over medium heat, melt the butter and introduce the minced garlic. Sauté for 1 to 2 minutes until the garlic oils are fragrant but not browned to avoid a bitter profile.

Brush the warm garlic butter liberally across the cut side of both bread halves.

Place the bread into the oven and toast until the edges demonstrate initial browning and a crisp texture, approximately 3 to 4 minutes.

In a medium vessel, combine the grated cheddar, Monterey Jack, Parmesan, mayonnaise, green onions, and salt.

Stir until a thick, homogenous lipid emulsion is achieved.

Apply the cheese emulsion evenly across the warm, toasted bread halves, spreading to the edges to seal the crumb.

Return to the 425°F oven and bake until the topping reaches a molten state, initiating an active "ooze and bubble," approximately 8 to 10 minutes.

Slice the bread into uniform wedges and serve immediately while the lipids are thermally active. To maintain temperature for a crowd, wrap the bread loosely in aluminum foil.