Dressing: Add all the dressing ingredients to a mason jar. Zest the lemon to get 1/2 tsp zest and juice it for 3 tbsp juice. Season with 1/2 tsp salt and 1/4 tsp pepper (or to taste). Seal the jar and shake well. Chill in the fridge while you prepare the salad.
Bring 12 cups of water to a rolling boil. Add 1 tablespoon salt, then 1 cup orzo. Cook according to package directions until al dente. Drain the pasta, rinse under cold water for 20 seconds, and shake off any extra water. Transfer the pasta to a large bowl.
Shake the dressing again and pour about 1/3 of it over the pasta. Toss to coat evenly, then refrigerate until fully cooled.
Add drained tuna, roasted peppers, olives, parsley, red onion, cucumber, and feta to the bowl with the pasta.
Drizzle on as much dressing as you like. You probably won't use it all. As the salad sits out, it can soak up the dressing, so you might want to add a little more later. Gently toss and enjoy right away. This salad doesn’t store well once dressed.
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Notes
Note 1: If you're sensitive to raw onion, soak the diced onion in salted ice water for 10 minutes. Drain well before adding.Storage: Once dressed, the salad doesn’t store well—the veggies soften and the orzo bloats. For make-ahead, keep everything separate and toss with dressing before eating. For meal prep, store the salad without dressing and keep onions, olives, and feta separate. Let leftover dressing sit at room temp for 20 minutes, then shake well.