For the dough: Mix the flour and butter. Add eggs and water, and combine until mixture is doughy. Form dough into 6 equal-sized balls. Use the dough immediately or let it rest in the refrigerator for about 30 min. if it is too warm to work with. Flatten balls into disks, then roll them out on a lightly floured surface, creating 6 small circles of dough. Bake at 350°F for 15 min. or till tarts are lightly browned. To make the topping, start by melting the butter in a frying pan. Add onion and thyme; sauté on low heat. (To quicken the caramelization of the onion, add 1/2 cup of water.) Keep stirring until onions are soft and browned, not burnt. Remove from heat and add to tart, warm or cold. Top with sour cream.