In a small bowl, combine the flour and butter. Using a fork, work it into a paste. (This paste is called buerre manie). At this point you can refrigerate it until needed. In a small saucepan combine the wine, water, shallots, bay leaf, salt and a generous pinch of white pepper. Bring this mixture to a boil. Add the shrimp and stir. Allow the liquid to come back to a boil. As soon as the shrimp turn pink, remove them from the liquid and place on a warm plate. Bring the poaching liquid back to a rolling boil. While whisking the hot liquid, add 1/2 of the buerre manie. As the buerre manie melts, the liquid will thicken. (If you want a thicker sauce, add more). Turn down the heat and simmer for 5 minutes, stirring often. Add the tomatoes and fresh thyme. Adjust the seasonings and ladle the sauce over the shrimp. The shrimp should be nicely mounded on a warm service plate. Garnish with thyme sprigs.