Preheat oven to 400 degrees F. On a large baking sheet, place 1 sheet filo pastry and brush with melted butter, repeat, stacking them on top of each other as you build your pie crust. You may want to stagger the sheets a little bit to ensure that the baking sheet is well covered allowing a little bit of an edge. On the top layer, brush on the mustard. Sprinkle the cheese evenly over the pastry to ensure even coverage. Lay the tomato slices generously, all over, overlapping, as needed. Season with coarse salt, and ground black pepper. Sprinkle the thyme leaves on top. Bake in the oven until the pastry is crisped and browned at the edges, about 25 minutes. Add another sprinkling of coarse salt, and cut the tart into 8 squares. Garnish with fresh basil leaves. Delicious when served hot, warm, or at room temperature.