- With the rack in the middle position, preheat the oven to 200 °C (400 °F). Line a baking sheet with parchment paper.
- On a lightly floured surface, roll out the dough to about 3-mm (1/8-inch) thick. With a 4.5-cm (1 ¾-inch) round cookie cutter, make enough circles to match your quantity of snails. With a fork, prick the centre of the dough several times (see note). Place on the baking sheet and refrigerate.
- Cut the tomato caps. Cut a thin slice at the base so they stay in place. Remove the flesh with a small melon baller. Set aside.
- In a skillet, melt the butter with the garlic. Add the snails and cook for about 1 minute while stirring. Add the chives. Season with pepper. Stuff each tomato with a snail. Set aside.
- In a bowl, combine the sour cream and mustard. Brush over each circle of dough. Top with the stuffed tomatoes. Drizzle with the remaining garlic butter. Bake until golden brown, about 20 minutes. Season with salt and pepper. Serve lukewarm as an hors d’oeuvre.
Prick the dough thoroughly, because the tomatoes can easily fall on the side while baking. Simply place them back when arranging them on a serving platter.