Grate potato on large hole side of box grater. Place grated potatoes in colander and rinse thoroughly. Transfer potatoes to a clean kitchen towel and wring out as much liquid as possible. Drying potatoes thoroughly are key to crispy hash browns. Transfer potatoes to a bowl and toss with garlic salt and cornstarch.
In a large nonstick pan over medium heat, add 2 tablespoons olive oil. Divide potato mixture into 4 equal portions and add to the hot pan, pressing into even pieces. Cook, undisturbed, 5 minutes, or until undersides are golden brown. Flip potatoes, add 2 tsp olive oil to pan, and cook undisturbed, 5 minutes, or until undersides are brown and potatoes are tender. Transfer to a plate.