Preheat broiler.
In a large Dutch oven over medium heat:
Add olive oil and sauté onion with a pinch of salt until soft and lightly golden, about 6–8 minutes.
Add tomato paste and 2 tablespoons taco seasoning.
Cook 2 minutes until paste darkens and smells toasty.
Taste the base. Add more taco seasoning if your blend is mild.
Deglaze with about 1 cup chicken stock, scraping up the bottom.
Add remaining stock and water (or up to 6 cups total stock).
Bring to a steady simmer.
Taste the liquid. It should be well-seasoned before adding pasta. Adjust with taco seasoning or salt if needed.
Stir in dry fusilli.
Simmer uncovered, stirring frequently, until pasta is just al dente and most liquid is absorbed, about 12–14 minutes.
Add small splashes of hot water if it tightens before tender.
Reduce heat to low.
Fold in shredded chicken and black beans. Warm through.
Taste again and adjust seasoning.
Remove from heat.
Stir in whole milk Greek yogurt or sour cream. Add lime juice if needed for brightness.
Top evenly with shredded cheese.
Broil 3–5 minutes until melted and lightly golden.
Finish with lots of scallions.