Heat the oil in a skillet over a medium to high heat. Add the garlic and thyme leaves, cook for about 2 minutes then remove the garlic. Carefully transfer the oil to a large bowl then add the bread. Mix with a large spoon so the bread is lightly coated in the oil. Spread the bread out on a baking tray and bake in the middle of the oven for 8 minutes.
Remove the tray from the oven then sprinkle the parmesan over the croutons, toss the croutons a little then return to the oven and bake for another 5 minutes. They should be crispy and golden brown. Remove from the oven sprinkle with the sea salt and leave to cool.
Keep any leftover croutons in an airtight container and use within a week.