Once the pie is fully chilled, in a large metal bowl whisk to combine the sugar and egg whites. The mixture will be very thick and grainy.
Put 1 inch / 2.5cm of water in the bottom of a medium saucepan and bring to a gentle simmer over medium-low heat. Place the bowl with the egg mixture over the simmering water and stir the mixture with a rubber spatula until the sugar is completely melted. Brush down the sides of the bowl with the spatula to make sure all the sugar is melted. Feel the mixture between your fingers to check for graininess. Once all the sugar has melted and the mixture is smooth, the syrup is hot enough (140°F / 60°C) to be safe to consume.
Pour the heated mixture into the bowl of a stand mixer fitted with the whisk attachment, add the Baking Salt, and beat on medium-high speed until the egg foam is light, fluffy, glossy, and the bowl feels just about room temperature.
Remove from the mixer and use a rubber spatula to gently stir in the lemon thyme leaves. Gently spoon the meringue over the chilled lemon filling. Using a metal decorating spatula, spread the meringue to the edges of the crust, being sure to cover all of the filling, and create decorative swirls in the meringue as you go.
Use a blow torch to toast the meringue and serve. The meringue can be spread over the pie and toasted a few hours before serving and stored, uncovered in the refrigerator. Use a sharp knife dipped in hot water to slice into the pie.