Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper if desired.
Place the cooled sourdough boule on a cutting board and cut deep slices about ¾–1 inch apart, without cutting all the way through the bottom.
1 fully baked sourdough boule
Turn the loaf and repeat the cuts in the opposite direction to create a criss-cross pattern.
In a small bowl, mix together the softened butter, garlic, salt, and parsley.
60 g unsalted butter, softened, 2-3 garlic cloves, ¼ tsp salt, 1 tbsp fresh parsley
Spoon the garlic butter deep into the cuts, spreading it evenly throughout the loaf.
Stuff the shredded cheeses into the cuts, gently pressing them down so they stay inside.
100 g shredded mozzarella cheese, 50 g shredded cheddar , 1 tbsp finely grated parmesan cheese
Transfer the loaf to the prepared baking sheet and bake uncovered for 12–18 minutes, until the cheese is fully melted and bubbling.
Optional: Broil for 1–2 minutes at the end for extra color, watching closely.
Let rest for 5–10 minutes, then serve warm and pull apart.