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