Arrange a rack in the middle of the oven. Heat to high broil. Line a rimmed sheet pan with foil or high-heat parchment paper.
Using a sharp serrated knife, cut the focaccia loaf in half horizontally. Place cut sides up on the prepared sheet pan. Spread softened butter all over the cut sides, then sprinkle lightly with garlic powder and salt.
Broil on the middle rack for 2 minutes, or until lightly toasted. Watch closely so it doesn’t burn. Remove from the oven and reduce oven temperature to 400ºF.
Sprinkle all three cheeses evenly over the bottom half of the bread. Place the top half back on to form a sandwich.
Return to the oven and bake 15–18 minutes, or until the bread is crisp and the cheese is fully melted.
Transfer to a cutting board, cool for 2 minutes, then cut crosswise into 6 sandwiches. If desired, gently lift the tops and spread with pesto. Serve warm and gooey!
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Notes
Note 1: Trader Joe’s loaf is amazing, but any thin, flavorful focaccia works. You can also use my homemade focaccia recipe by making the thinner version in a 13×18-inch pan (see recipe card). If your loaf is larger, add extra cheese to match.Note 2: Shred cheese yourself on the large holes of a box grater for the best flavor and texture. Gently pack it into the measuring cup to ensure you have the right amount.Storage: Store leftovers in an airtight container in the fridge for 1–2 days. Reheat in the oven until warm and melty.