Cut the bread into approximately 1 1/2-inch cubes. Set aside.
In a large bowl, mix the lemon zest, lemon juice, garlic and salt together. Add the oil and use a whisk to mix until it's well blended.
Add the bread cubes to the bowl and toss until all of the bread is well coated with the mixture. (This is easiest if you use your hands.)
Preheat the oven to 400°F, and let the bread marinate for 10 minutes.
Spread the marinated bread onto a baking sheet, in one layer. (Do not cover the baking sheet with anything beforehand.)
Grind a bit of black pepper over the bread, and then roast in the preheated 400°F oven until the bottoms of the bread cubes are golden brown, about 10 minutes. Then turn them over and roast until the other side browns, another 5 minutes or so.
Let the croutons cool on the baking sheet before using them.