Preheat the broiler.
Combine the garlic, oil, butter, and herb seasoning in a rimmed baking sheet. Place the sheet in the oven and keep it there just until the butter melts, 1 to 2 minutes. Stir to combine the ingredients.
Cut the bread in half through the side to make two wide slabs. Dip each slab, cut side down, into the garlic mixture, pressing on the bread until it absorbs the liquid and seasoning. Broil the slabs cut side up on the baking sheet until golden brown on top, 3 to 4 minutes. Cut them into 1-inch cubes and set the pieces aside.
Meanwhile, place the cheeses and cornstarch in a large bowl or zip-top bag and toss to combine.
Bring 1¼ cups of the wine to a simmer in a fondue pot or a small, heavy-bottom saucepan over medium heat. Gradually stir in the cheese mixture until melted and smooth, 3 to 4 minutes. Stir in the kirsch or grappa and cook for 1 to 2 minutes. If the fondue is too thick, stir in the remaining wine 1 tablespoon at a time. Keep the fondue just under a simmer over the lowest possible heat. Serve it with bread cubes on forks for dipping, stirring each time you dip to keep the mixture smooth.
Tip: Cubes of Granny Smith apple (with skin) make a delicious accompaniment to the bread. Toss the apple cubes in lemon juice to prevent browning.
For a nonalcoholic version, replace the wine with 1½ cups of 2% milk, and the kirsch or grappa with 2 tablespoons of lemon juice.