Cook the pasta: Bring a large pot of salted water to a boil. Cook the pasta until just al dente according to package directions.
Blanch the broccoli: In the last 60–90 seconds of the pasta cooking time, add the broccoli florets to the pot.
They should turn bright green and slightly tender.
Drain and cool: Drain pasta and broccoli together. Rinse briefly under cold water to stop cooking and cool them down. Shake off excess water well.
Make the dressing: In a large bowl, whisk Greek yogurt, mayonnaise, Dijon, lemon zest and juice, olive oil, garlic powder, onion powder, 3/4 teaspoon salt, and 1/2 teaspoon pepper.
Adjust to taste.
Add the mix-ins: Fold in chicken, cheddar, red onion, and celery. Stir to coat lightly with the dressing.
Combine: Add the cooled pasta and broccoli to the bowl. Toss gently until everything is evenly coated and glossy.
If it seems dry, add a splash of olive oil or a spoon of yogurt.
Finish and chill: Stir in fresh herbs. Taste and adjust salt, pepper, or lemon. Cover and chill for at least 30 minutes to let flavors meld.
Serve: Give it a quick toss before serving.
Add a sprinkle of cheddar or herbs on top for a fresh look.