1. Wash the potatoes and dry them while keeping the skin. Using a mandoline or a knife, slice them into regular rounds about 3 mm thick.
2. On a baking sheet lined with parchment paper, place a round piece of parchment paper (which will serve as a guide) and arrange the potato slices on top, overlapping them nicely to form a "tart base". There should be no space between each slice. Lightly salt.
3. Preheat the oven to 180 °C (350 °F). In a bowl, mix the mustards and brush them onto the potatoes using a brush.
4. Slice the tomatoes into regular rounds. Arrange them on the "tart base", overlapping them as well. Drizzle olive oil on top. Lightly salt, pepper, and sprinkle with cumin.
5. Bake for 40 minutes: the tomatoes should be well roasted and the edges crispy. Serve warm as a side dish with grilled meat or fish.