Using a serrated knife, slice about 1/8 inch from the top of each tomato. If tomatoes do not sit upright, trim a tiny piece from the uncut side to create a flat bottom. (Do not cut off too much or filling will leak out.) With the tip of a small knife, gently loosen the flesh around the inside of each tomato. Scoop out seeds with a small melon baller. Tomatoes can be prepared 1 hour in advance and kept at room temperature.