Preheat oven to 400 degrees F. Lightly flour a baking sheet.
In a bowl, whisk together yolk and sour cream, season with salt and pepper. Arrange pastry squares on baking sheet and spoon 1/4 of mixture onto center of each. Top mixture with artichokes and tomatoes. Spoon remaining sour cream mixture over vegetables and sprinkle with thyme.
Bake tarts in middle of oven until pastry is golden, about 12 minutes.
Recipe courtesy of Gourmet Magazine