Place the celery root, parsnips, potatoes and garlic into a large pot. Cover with cold water by an inch and add 2 tablespoons of salt. Place over high heat and bring to a boil. Reduce the heat to medium and simmer for about 10 to 15 minutes or until all of the vegetables are tender. Drain well and return the cooked vegetables to the pot. Using a wooden spoon, stir the vegetables over medium heat to remove some of the excess moisture. This will take about 4 minutes. Using a large fork or a potato masher, lightly mash the vegetables. Remove the smashed and dried vegetables from the heat and add the remaining 1 teaspoon salt, the lemon zest, thyme, olive oil and cheese. Mix until well combined. Serve hot.