11. Preheat oven to 350 degrees.
2. Without cutting through the bottom crust, gently slice sourdough loaf lengthwise and widthwise, making a grid pattern (leave about 1/2 inch on bottom of loaf unsliced).
3. Place loaf on a foil-lined baking sheet.
4. Insert cheese slices between the cuts of bread.
5. Combine butter, onion, and garlic powder; drizzle over bread.
6. Wrap entire loaf in foil; place on a baking sheet.
7. Bake for 15 minutes; unwrap the bread and bake 10 more minutes, or until cheese is melted.