Blitz the basil, 75 millilitres olive oil and garlic together in a food processor until fine.
Spread the basil oil on all 4 slices of sourdough then divide the mozzarella and tomato slices between 2 of the slices. Top with the remaining 2 slices to form 2 sandwiches.
Drizzle both toasties with 1 tablespoon of the remaining olive oil and place into a large frying pan. Cook for 3 - 4 minutes each side, drizzling with the remaining 1 tablespoon olive oil when you turn over, until the sandwiches are golden and melted throughout.