To prepare the dressing, add the olive oil, vinegar, lemon juice, mustard, garlic, oregano, salt, and black pepper to a mason jar with a lid. Stir or shake very well, until the dressing looks well mixed. Set it aside as you assemble the salad.
To prepare the salad, add the chopped romaine, radicchio (if using), red onion, tomatoes, pepperonchi, and Parmesan to a large bowl. Add a generous drizzle of dressing, and toss well to combine.
Taste the salad and add more dressing, to taste. You might not use all of the dressing, and in that case you can save it in the fridge for a future salad. Add the croutons shortly before serving, so they won't get soggy. (Or use chickpeas for a protein-rich swap.) Serve the salad right away for the best taste and texture.
Leftover veggies and dressing can be stored separately in airtight containers in the fridge for up to 5 days. I don't recommend storing the salad for too long after adding the dressing, as the lettuce will become soggy quickly. (So will the croutons!)
