Preheat the oven to 375°F.
Place a baking sheet in the oven to preheat.
Lay out a large sheet of foil on the counter.
Place the sourdough loaf on the foil.
In a bowl, mash the softened butter with the olive oil until glossy.
Stir in the minced garlic, parsley, flaky salt, black pepper, and garlic powder if using.
Fold in the grated Parmesan if using.
Stir in the lemon zest if using.
Cut deep slits into the loaf about 1 inch apart without cutting through the bottom crust.
Use a butter knife to tuck the garlic butter mixture into every slit.
Press a little of the garlic butter mixture over the top of the loaf.
Wrap the foil loosely around the loaf, leaving space above it.
Carefully place the foil-wrapped loaf on the preheated baking sheet.
Bake for 15 minutes.
Open the foil so the top of the loaf is exposed.
Bake for 8 to 12 minutes more, until the edges look deep golden.
Let the loaf rest for 5 minutes before serving.
Taste and add more flaky salt if needed.