Pull the thyme leaves off the stems and set the leaves aside. Discard the stems.
Arrange the chicken tenderloins on a large sheet pan. Season with half the salt and pepper.
Drizzle olive oil over the chicken, then sprinkle on the thyme and half the garlic powder.
Squeeze the juice from half a lemon over the chicken.
Add the broccoli and cauliflower florets around the chicken. Sprinkle with the remaining salt, pepper, and garlic powder, then squeeze juice from another half lemon over the veggies.
Slice the remaining lemon into thin rounds and arrange across the pan.
Bake 20 minutes and check doneness of chicken. If the cauli is not al dente but the chicken has reached 165°F, remove the chicken with tongs and let the veg roast another 6 to 8 minutes
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