In bowl, combine chicken, apple, cheddar, green onion and thyme.
Divide the mixture over half (12) of the tart shells (scant ¼ cup per tart) Brush edges of tarts with egg. Gently remove remaining tart shells from their foil and top each to make pies, using the tines of a fork to secure edges.
Brush with egg and sprinkle each with coarse salt. Using the tip of a sharp knife, make a few ventilation slits in the top of each pie.
Bake at 400°F (200°C) until pastry is golden and filling is hot and bubbling, about 30 minutes. Let cool for 5 minutes before serving.