Prep Tomatoes: Cut each tomato in half, width wise. If using smaller tomatoes, simply cut a small portion off the top and bottom so they stand upright and have the tops exposed.
Mustard: Evenly brush the top of each tomato with mustard.
Filling: In a medium bowl, stir together all remaining ingredients.
Mound: Mound the breadcrumb filling onto each tomato, then place them on a baking sheet.
Broil: Set your oven to broil and set the tomatoes on the middle rack. Broil, watching closely, until the tops are golden brown, 3 to 5 minutes.