Cook the pasta. Bring a large pot of salted water to a boil. Cook the pasta until al dente according to package directions. Reserve 1 cup of pasta water, then drain.

Simmer the cauliflower. Add cauliflower florets to a pot with the chicken broth. If needed, add water to barely cover. Bring to a simmer, cover, and cook until very tender, about 10–12 minutes.

Sauté the garlic. While the cauliflower cooks, heat 1 tablespoon olive oil in a large skillet over medium heat. Add garlic and cook for 30–60 seconds until fragrant. Do not brown. Transfer the garlic to your blender or food processor.

Blend the sauce. Using a slotted spoon, transfer the tender cauliflower to the blender with the garlic. Add about 1 cup of the hot cooking liquid. Add Parmesan, onion powder, a pinch of salt, and pepper. Blend until very smooth and creamy. Add more broth or a splash of pasta water to reach your desired thickness. Taste and adjust seasoning with salt, pepper, and lemon juice.

Cook the chicken. Pat the chicken dry and season both sides with salt and pepper. In the same skillet, heat the remaining 1 tablespoon olive oil over medium-high. Sear chicken 3–4 minutes per side until golden and cooked through. Remove to a plate and slice.

Finish the sauce in the skillet. Lower the heat to medium-low. If using butter, add it to the skillet and let it melt. Pour in the cauliflower sauce and stir, scraping up any browned bits from the pan. Add lemon zest and red pepper flakes if using. Warm gently; avoid boiling.

Toss with pasta. Add cooked pasta to the skillet and toss until coated. Use reserved pasta water to loosen if needed. Fold in the sliced chicken.

Serve. Top with extra Parmesan, a sprinkle of parsley, and freshly ground black pepper. Taste once more and add a squeeze of lemon if you want extra brightness.