Prep the Tomatoes: Start with the freshest tomatoes you can find. Rinse and dry them well. Slice them into wedges or thick rounds—whatever feels right. I love using a mix of colors for that vibrant, summery look.

Slice the Onions: Thinly slice the red onion and separate the rings. If the onion is strong, soak the slices in cold water for 5–10 minutes, then drain. This softens the bite while keeping the crunch.

Make the Marinade: In a bowl, whisk together olive oil, vinegar, lemon juice, Dijon mustard, minced garlic, sugar (if using), salt, pepper, and oregano. The mustard helps emulsify the dressing, creating a silky marinade that clings to every slice of tomato.

Combine and Toss: In a large bowl or shallow dish, combine the tomatoes and onions. Pour the marinade over them and gently toss to coat. Make sure every tomato slice is bathed in that flavorful dressing.

Let It Marinate: Cover and refrigerate for at least 30 minutes—or up to 2 hours—for best flavor. Give it a gentle stir before serving.

Serve and Garnish: Serve cold or at room temperature, garnished with chopped parsley or fresh basil for a fragrant finish.