Make the salad dressing by whisking together the mustard, lemon juice, (optional) lemon zest, dried oregano, a pinch of salt, a pinch of black pepper, and olive oil.
Add the cherry tomatoes (halved) to the bowl with the salad dressing and then add the chopped avocado, (optionally toasted) chopped pecans, and arugula.
Just before serving, toss everything together. Taste it and, if needed, add more salt and pepper.