Pat chicken breasts dry with paper towels and season both sides with salt and pepper. Heat olive oil in a large pot over medium-high heat. Sear chicken for 4-5 minutes per side until golden brown and the internal temperature reaches 165°F. Remove from pot, let rest for 5-10 minutes, then shred or dice into bite-sized pieces.

Reduce heat to medium and add butter to the same pot. Once melted, add diced onion, carrots, and celery. Sauté for 8-10 minutes until vegetables soften and the onion becomes translucent. Add minced garlic and cook for 1 minute until fragrant. Stir in thyme, smoked paprika, and dry mustard powder, cooking for another minute while stirring constantly.

Sprinkle flour over the vegetables and stir vigorously to coat everything evenly. Cook for 2-3 minutes, stirring continuously, until the mixture forms a pale blonde roux that smells nutty and looks like wet sand.

Gradually pour in chicken broth one cup at a time, whisking constantly after each addition to prevent lumps. Slowly add milk and heavy cream while whisking. Bring to a gentle simmer over medium heat, stirring frequently. Season with a pinch of salt and pepper.

Reduce heat to low. Add freshly grated cheddar and Monterey Jack cheeses in small handfuls, stirring constantly until each addition melts completely before adding more. Do not let the soup boil. Stir until the soup is completely smooth and creamy.

While the soup simmers, cook macaroni separately in a medium pot of generously salted boiling water according to package directions until al dente. Drain thoroughly.

Add the cooked chicken and drained macaroni to the cheese soup, stirring gently to combine. Let simmer uncovered on low for 5-10 minutes to allow flavors to meld. Taste and adjust seasonings, adding a splash of broth or milk if the soup becomes too thick.

Ladle into bowls and garnish with fresh parsley or chives if desired. Serve immediately.