Preheat the oven to 180°C (350°F). Line a large baking tray with parchment paper and set aside.
In a large mixing bowl, combine the rolled oats and protein powder and stir until evenly mixed.
In a microwave-safe bowl, add the peanut butter and maple syrup (or honey) and heat in 20-second intervals, stirring between each, until the peanut butter is melted and the mixture is smooth.
Pour the warmed peanut butter mixture into the dry ingredients and stir thoroughly until the oats and protein are fully coated and evenly combined.
Spread the mixture in an even layer on the prepared baking tray. Bake for 13–15 minutes, stirring once halfway through to promote even browning.
Remove the granola from the oven and stir to break up any large clumps. Let the granola cool completely on the tray until it reaches room temperature.
Once cooled, transfer the granola to a bowl and fold in 1 cup of your chosen mix-ins (for example dried cranberries, cashews, almonds, or banana chips).