Slice the tomatoes into wedges or thick rounds, depending on what looks best with the shape of your tomatoes. Arrange them on a serving plate.
Sprinkle the shallots, garlic and oregano evenly over the tomatoes. This way, their flavor gently infuses each slice—no need for a dressing bowl.
Drizzle with balsamic vinegar and extra virgin olive oil. Try to pour slowly and evenly over the tomatoes so each piece gets a bit of both.
Season with salt and freshly ground black pepper to taste. Let it sit for about 5–10 minutes to allow the flavors to soak in.
Just before serving, scatter the mâche over the top. You can gently toss everything together, or leave it layered.